In a non-image forming state, an image forming apparatus changes the magnitude of a control signal output by a control portion, and when a current supply member supplies a belt with an amount of current generated by adding, to a predetermined target current used for image formation, an amount of current that flows from a contact member to the ground and changes in accordance with the magnitude of the control signal, the image forming apparatus acquires the magnitude of the control signal generated when the current, which is supplied from the current supply member to the belt and flows to a voltage adjusting member from the belt via the contact member, becomes zero.