An image forming apparatus comprising: a plurality of image bearers; a plurality of image forming devices to form toner images on the plurality of image bearers, respectively; an endless belt including a plurality of layers including: a base layer; and at least one layer superimposed on the base layer in such way that it faces to the outside of the endless belt and can thereby contact the image bearers, wherein the endless belt is disposed in contact with the plurality of image bearers to form transfer nips; and a plurality of transfer bias members, to each of which a transfer bias is applied in a state in which the transfer bias member sandwiches the endless belt together with one of the plurality of image bearers to transfer a toner image from the one of the plurality of image bearers onto a surface of the endless belt or a recording medium conveyed by the endless belt in each of the transfer nips, wherein the plurality of transfer bias members includes an upstream transfer bias member and a downstream transfer bias member respectively located as the first transfer bias member and the last transfer bias member in a sequence of transfer process, and wherein the downstream transfer bias member is lower in electrical resistance than the upstream transfer bias member, characterized by a power supply configured to output, under constant-current control, a first transfer bias applied to the downstream transfer bias member and a second transfer bias applied to at least one of the rest (9Y; 9M; 9C) of the plurality of transfer bias members; an environment detector to detect temperature and relative humidity in the image forming apparatus; and a controller to correct a first target value of the first transfer bias and a second target value of the second transfer bias, wherein the power supply is configured to output the second transfer bias separately from the first transfer bias, wherein the controller is configured to correct the first target value and the second target value based on a detection result generated by the environment detector, and wherein the controller is configured to correct the first target value and the second target value according to different algorithms. a plurality of image bearers; a plurality of image forming devices to form toner images on the plurality of image bearers, respectively; an endless belt including a plurality of layers including: a base layer; and at least one layer superimposed on the base layer in such way that it faces to the outside of the endless belt and can thereby contact the image bearers, wherein the endless belt is disposed in contact with the plurality of image bearers to form transfer nips; and a base layer; and at least one layer superimposed on the base layer in such way that it faces to the outside of the endless belt and can thereby contact the image bearers, wherein the endless belt is disposed in contact with the plurality of image bearers to form transfer nips; and a plurality of transfer bias members, to each of which a transfer bias is applied in a state in which the transfer bias member sandwiches the endless belt together with one of the plurality of image bearers to transfer a toner image from the one of the plurality of image bearers onto a surface of the endless belt or a recording medium conveyed by the endless belt in each of the transfer nips, wherein the plurality of transfer bias members includes an upstream transfer bias member and a downstream transfer bias member respectively located as the first transfer bias member and the last transfer bias member in a sequence of transfer process, and wherein the downstream transfer bias member is lower in electrical resistance than the upstream transfer bias member, characterized by a power supply configured to output, under constant-current control, a first transfer bias applied to the downstream transfer bias member and a second transfer bias applied to at least one of the rest (9Y; 9M; 9C) of the plurality of transfer bias members; an environment detector to detect temperature and relative humidity in the image forming apparatus; and a controller to correct a first target value of the first transfer bias and a second target value of the second transfer bias, wherein the power supply is configured to output the second transfer bias separately from the first transfer bias, wherein the controller is configured to correct the first target value and the second target value based on a detection result generated by the environment detector, and wherein the controller is configured to correct the first target value and the second target value according to different algorithms.