An image forming apparatus comprising: a photosensitive member (50a, 50b, 50c, 50d); an image forming portion (101a, 101b, 101c, 101d) for forming an electrostatic image on the photosensitive member to deposit a toner image on an image portion of the electrostatic image; an intermediary transfer member for carrying the toner image primary-transferred from the photosensitive member at a primary-transfer position (N1a, N1b, N1c, N1d); and a transfer member, provided contactable to an outer peripheral surface of the intermediary transfer member, for secondary-transferring the toner image from the intermediary transfer member onto a recording material at a secondary-transfer position, characterized by 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; 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; a detecting member for detecting an ambient condition; and a controller for controlling a voltage to be applied to said transfer member by said power source so that the passive constant-voltage element maintains the predetermined voltage, and for controlling the image forming portion to change a potential difference between the predetermined voltage and the potential of the image portion depending on a detection result of the detecting member. a photosensitive member (50a, 50b, 50c, 50d); an image forming portion (101a, 101b, 101c, 101d) for forming an electrostatic image on the photosensitive member to deposit a toner image on an image portion of the electrostatic image; an intermediary transfer member for carrying the toner image primary-transferred from the photosensitive member at a primary-transfer position (N1a, N1b, N1c, N1d); and a transfer member, provided contactable to an outer peripheral surface of the intermediary transfer member, for secondary-transferring the toner image from the intermediary transfer member onto a recording material at a secondary-transfer position, characterized by 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; 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; a detecting member for detecting an ambient condition; and a controller for controlling a voltage to be applied to said transfer member by said power source so that the passive constant-voltage element maintains the predetermined voltage, and for controlling the image forming portion to change a potential difference between the predetermined voltage and the potential of the image portion depending on a detection result of the detecting member.