An image forming apparatus, comprising: a rotatable intermediate transfer belt; a first image carrier (2a, 2b, 2c) and a second image carrier each configured to carry a toner image, wherein the first image carrier and the second image carrier are located in a confronting relationship with the intermediate transfer belt and disposed along a rotational direction of the intermediate transfer belt; a first transfer member (14a, 14b, 14c) arranged on an opposite side of the intermediate transfer belt from the first image carrier (2a, 2b, 2c); and a second transfer member arranged on an opposite side of the intermediate transfer belt from the second image carrier; wherein the image forming apparatus is configured to: be switchable between a multicolor mode and a mono-color mode, cause, in the multicolor mode, the first image carrier (2a, 2b, 2c) and the first transfer member to be urged towards each other to allow transfer of toner images from the first image carrier (2a, 2b, 2c) onto the intermediate transfer belt, and the second image carrier and the second transfer member to be urged towards each other to allow transfer of toner images from the second image carrier onto the intermediate transfer belt, and then the intermediate transfer belt to secondarily transfer the superimposed toner images to a transfer material, and cause, in the mono-color mode, the second image carrier and the second transfer member to be urged towards each other to allow transfer of toner images from the second image carrier onto the intermediate transfer belt in a state where the first transfer member (14a, 14b, 14c) is separated from the intermediate transfer belt, and then the intermediate transfer belt to secondarily transfer the transferred toner image to the transfer material(s), characterized in that the image forming apparatus is configured to: change a rotational speed of the intermediate transfer belt to a first rotational speed slower than a second rotational speed of the intermediate transfer belt used in the multicolor mode during transfer of the toner image to the intermediate transfer belt in a case that the operational mode is switched from the multicolor mode to the mono-color mode; separate, after a rotational speed of the intermediate transfer belt is changed to the first rotational speed, the first transfer member (14a, 14b, 14c) from the intermediate transfer belt that is rotating at the first rotational speed; and increase, after the first transfer member (14a, 14b, 14c) is separated from the intermediate transfer belt, the rotational speed of the intermediate transfer belt to the second rotational speed. a rotatable intermediate transfer belt; a first image carrier (2a, 2b, 2c) and a second image carrier each configured to carry a toner image, wherein the first image carrier and the second image carrier are located in a confronting relationship with the intermediate transfer belt and disposed along a rotational direction of the intermediate transfer belt; a first transfer member (14a, 14b, 14c) arranged on an opposite side of the intermediate transfer belt from the first image carrier (2a, 2b, 2c); and a second transfer member arranged on an opposite side of the intermediate transfer belt from the second image carrier; be switchable between a multicolor mode and a mono-color mode, cause, in the multicolor mode, the first image carrier (2a, 2b, 2c) and the first transfer member to be urged towards each other to allow transfer of toner images from the first image carrier (2a, 2b, 2c) onto the intermediate transfer belt, and the second image carrier and the second transfer member to be urged towards each other to allow transfer of toner images from the second image carrier onto the intermediate transfer belt, and then the intermediate transfer belt to secondarily transfer the superimposed toner images to a transfer material, and cause, in the mono-color mode, the second image carrier and the second transfer member to be urged towards each other to allow transfer of toner images from the second image carrier onto the intermediate transfer belt in a state where the first transfer member (14a, 14b, 14c) is separated from the intermediate transfer belt, and then the intermediate transfer belt to secondarily transfer the transferred toner image to the transfer material(s), change a rotational speed of the intermediate transfer belt to a first rotational speed slower than a second rotational speed of the intermediate transfer belt used in the multicolor mode during transfer of the toner image to the intermediate transfer belt in a case that the operational mode is switched from the multicolor mode to the mono-color mode; separate, after a rotational speed of the intermediate transfer belt is changed to the first rotational speed, the first transfer member (14a, 14b, 14c) from the intermediate transfer belt that is rotating at the first rotational speed; and increase, after the first transfer member (14a, 14b, 14c) is separated from the intermediate transfer belt, the rotational speed of the intermediate transfer belt to the second rotational speed.