An image forming apparatus comprising: an image bearing member (1a, 1b, 1c, 1d) for bearing a toner image; an intermediary transfer member for carrying the toner image transferred from the image bearing member at a primary-transfer position (N1a, N1b, N1c, N1d); a transfer member, provided contactable to an outer peripheral surface of the intermediary transfer member, for transferring the toner image from the intermediary transfer member onto a recording material at a secondary-transfer position; a power source for forming, by applying a voltage to the transfer member, both of a secondary-transfer electric field at the secondary-transfer position and a primary-transfer electric field at the primary-transfer position; and a passive constant-voltage element, electrically connected between the intermediary transfer member and a ground potential, which is configured to maintain a predetermined voltage when a current passes through the passive constant-voltage element by applying a voltage to said transfer member by said power source, characterized by a detecting portion for detecting a current passing through the transfer member; an executing portion for executing a test mode in which, when no recording material exists at the secondary-transfer position, a test voltage is applied to the transfer member by the power source by constant-voltage control to detect the current by the detecting portion; and a controller for controlling, on the basis of the current detected by the detecting portion in the test mode, a voltage to be applied to the transfer member by the power source when the recording material exists at the secondary-transfer position, wherein the controller is configured to control the test voltage applied by the power source by constant-voltage control so that the passive constant-voltage element maintains the predetermined voltage in at least an overlapping period between a period of the test mode and a period in which the toner image is transferred at the primary-transfer position. an image bearing member (1a, 1b, 1c, 1d) for bearing a toner image; an intermediary transfer member for carrying the toner image transferred from the image bearing member at a primary-transfer position (N1a, N1b, N1c, N1d); a transfer member, provided contactable to an outer peripheral surface of the intermediary transfer member, for transferring the toner image from the intermediary transfer member onto a recording material at a secondary-transfer position; a power source for forming, by applying a voltage to the transfer member, both of a secondary-transfer electric field at the secondary-transfer position and a primary-transfer electric field at the primary-transfer position; and a passive constant-voltage element, electrically connected between the intermediary transfer member and a ground potential, which is configured to maintain a predetermined voltage when a current passes through the passive constant-voltage element by applying a voltage to said transfer member by said power source, characterized by a detecting portion for detecting a current passing through the transfer member; an executing portion for executing a test mode in which, when no recording material exists at the secondary-transfer position, a test voltage is applied to the transfer member by the power source by constant-voltage control to detect the current by the detecting portion; and a controller for controlling, on the basis of the current detected by the detecting portion in the test mode, a voltage to be applied to the transfer member by the power source when the recording material exists at the secondary-transfer position, wherein the controller is configured to control the test voltage applied by the power source by constant-voltage control so that the passive constant-voltage element maintains the predetermined voltage in at least an overlapping period between a period of the test mode and a period in which the toner image is transferred at the primary-transfer position.