Drum unit and image formation apparatus

A drum unit comprising:

• a photosensitive drum;
• and a frame that holds the photosensitive drum, wherein
• an equation of ΔV≤0.489×Q−0.014(%) is satisfied, where ΔV={1−(V1/V0)}×100(%),
• wherein V0 is a surface potential of the photosensitive drum that is measured under a first condition, the first condition being to expose a surface of the photosensitive drum at an exposure amount Q in a range of 0.13 to 0.34 μJ/cm2 while the photosensitive drum is rotated at a circumferential speed of 151 mm/s with the surface of the photosensitive drum being charged with a voltage having an absolute value of 1000 volts and a first polarity, and wherein V1 is a surface potential of the photosensitive drum that is measured under the first condition again after the surface of the photosensitive drum that has been measured under the first condition is applied for 406 seconds with a voltage having an absolute value of 2850 volts and a second polarity opposite to the first polarity while the photosensitive drum is rotated at the circumferential speed of 73 mm/s.