[0061]FIG. 6A is a schematic side v

[0061]FIG. 6A is a schematic side view indicating the biasing direction and the respective positions of the photosensitive drum and the charging roller in the first embodiment of the present invention. FIG. 6B is a schematic front view indicating the photosensitive drum and the charging roller in FIG. 6A.[0062]In the first embodiment of the present invention as described above, the charging roller shaft 50a is located at a position slightly displaced from the position directly below the photosensitive drum shaft 13a, and the charging roller shaft 50a is biased directly upward. That is, biasing force from the biasing member 80 (i.e. a second biasing force F2) functions in the direction intersecting with the first auxiliary line HL1.[0063]In this embodiment also, the charging roller 50 is bent as shown in FIG. 6B. However, since the direction in which the charging roller shaft 50a and the photosensitive drum shaft 13a face each other (i.e. the direction of the first auxiliary line HL1) is displaced from the direction in which the warp is generated (i.e. the direction of the second biasing force F2), the gap generated by the warp is reduced.[0064]In this embodiment as described above, the charging roller 50 is not biased straightly toward the central axis of the photosensitive drum 13. Thus, it is possible to reduce displacement of the contact position to the photosensitive drum 13 caused by distortion of the shaft of the charging roller 50. Thus, it is possible to uniformly charge the photosensitive drum 13 without reducing transmission efficiency of the pressure contact force to the charging roller 50.[0065]In FIG. 4, the biasing member 80 applies biasing force in the direction of the height direction Z. However, the present invention is not limited thereto. It is sufficient that the respective positions of the photosensitive drum shaft 13a, the charging roller shaft 50a and the cleaning roller shaft 60a are arranged such that the central axis line TL and the first auxiliary line HL1 intersect with each other. That is, the charging roller shaft 50a may be located at a position directly below the photosensitive drum shaft 13a, and in this case, the biasing member 80 is arranged to apply biasing force in the direction inclined with respect to the height direction Z.[0066]It is preferable that the charging roller 50 and the cleaning roller 60 are disposed under the photosensitive drum 13 in the height direction Z. With this configuration, even when the shaft bends downward by its own weight, the constant nip width can be maintained because the charging roller 50 and the cleaning roller 60 are biased upward, which is a direction displaced from the central axis of the photosensitive drum 13.[0067]Also, by adopting the charging roller 50 having a crown shape, it is possible to reduce the distortion of the shaft and to maintain the constant nip width. However, the present invention is not limited thereto. The charging roller 50 may have a straight cylinder shape having a constant diameter.[0068]As shown in a variation shown in FIG. 7, the biasing member 80 may be disposed such that the central axis line TL passes through a side end of the biasing member 80. It is sufficient that the center of the cleaning roller shaft 60a is on the central axis line TL while the biasing force of the biasing member 80 is applied to the cleaning roller 60. In other words, when the central axis line TL passes through the biasing member 80 so as to be parallel with the biasing direction, it can be considered that the biasing direction of the biasing member 80 is on the same line as the central axis line TL. The load from biasing member 80 may be adjusted by the bearing part 70 so as to be applied to the charging roller shaft 50a and the cleaning roller shaft 60a.Second Embodiment[0069]Hereinafter, an image forming apparatus (process unit) according to the second embodiment of the present invention will be described with reference to the drawings. Since the configuration of the image forming apparatus according to the second embodiment is substantially the same as that in the first embodiment, the common configuration is indicated by the same reference numerals, and the description and the drawing thereof are omitted.[0070]FIG. 8 is a schematic front view illustrating the process unit according to the second embodiment of the present invention. For the sake of understandability of the structure, the housing 41, a charging roller shaft bearing part 71 and a cleaning roller shaft bearing part 72 are indicated by the hatched lines in FIG. 8.