An image forming apparatus includes a rotatable photosensitive member, a charging unit, a developing unit, a support frame, and a sensor unit. The sensor unit is detachably attached to the support frame in a direction that intersects a rotation-axis direction of the photosensitive member and includes a potential sensor, a support portion, a fixing portion, an electric board, and a linking portion. The support portion supports the potential sensor at an area from the charging unit to the developing unit in a rotational direction of the photosensitive member. The fixing portion is fixed to the support frame at a position that faces an area from the developing unit to the charging unit in the rotational direction of the photosensitive member. The support portion is positioned with respect to the support frame at a position downstream of the support portion in an attachment direction of the sensor unit.