In an image forming apparatus having a photoconductive member supported via two supporting members fixed on a rotation axis and a flywheel with great moment of inertia, the characteristic oscillation frequency of the revolution system comprising the rotation axis, supporting members and photoconductive member is set so that any density unevenness on a toner image which is formed on the photoconductive member is not apparent even if an exciting torque affects the photoconductive drum. The moment of inertia of a rotation axis, a photoconductive drum, and a flywheel, a torsional regidity of the rotation axis and a mounting position of a support member, and a surface speed of the photoconductive drum all satisfy a specific relationship.