An image forming apparatus having: at least one inserting groove (22,23,24,25); a waste toner collecting device provided with at least one supporting rail (61,62,63,64), and at least one developing device, the or each developing device being detachably mountable to the image forming apparatus, wherein the or each developing device comprises: a cartridge body (51,53,55,57) having a front portion provided with a side part adapted to be press-fitted into the inserting groove (51a,53a,55a,57a) of the image forming apparatus, when the or each developing device is mounted in the image forming apparatus via the or each supporting rail (61,62,63,64) of the waste toner collecting device and comprising a pair of position fixing protrusions (51a,53a,55a,57a) at the rear part; a developing roller (52,54,56,58) rotatably mounted to the cartridge body (51,53,55,57); and a terminal (52a,54a,56a,58a) positioned on the cartridge body (51,53,55,57) such that the terminal establishes an electrical connection with an electrical connector (26,27,28,29) provided on the or each inserting groove (22,23,24,25), when the or each developing device is mounted in the image forming apparatus wherein the or each supporting rail (61,62,63,64) further comprises a first position fixing groove (61a,62a,63a,64a), the image forming apparatus further comprises at least one mounting rail (16,17,18,19) having a second position fixing groove (16a,17a,18a,19a), and the or each pair of position fixing protrusions (51a,53a,55a,57a) are adapted to be inserted into the or each first and second position fixing grooves (61a,62a,63a,64a and 16a,17a,18a,19a) when the or each developing device is mounted in the image forming apparatus. at least one inserting groove (22,23,24,25); a waste toner collecting device provided with at least one supporting rail (61,62,63,64), and at least one developing device, the or each developing device being detachably mountable to the image forming apparatus, wherein the or each developing device comprises: a cartridge body (51,53,55,57) having a front portion provided with a side part adapted to be press-fitted into the inserting groove (51a,53a,55a,57a) of the image forming apparatus, when the or each developing device is mounted in the image forming apparatus via the or each supporting rail (61,62,63,64) of the waste toner collecting device and comprising a pair of position fixing protrusions (51a,53a,55a,57a) at the rear part; a developing roller (52,54,56,58) rotatably mounted to the cartridge body (51,53,55,57); and a terminal (52a,54a,56a,58a) positioned on the cartridge body (51,53,55,57) such that the terminal establishes an electrical connection with an electrical connector (26,27,28,29) provided on the or each inserting groove (22,23,24,25), a cartridge body (51,53,55,57) having a front portion provided with a side part adapted to be press-fitted into the inserting groove (51a,53a,55a,57a) of the image forming apparatus, when the or each developing device is mounted in the image forming apparatus via the or each supporting rail (61,62,63,64) of the waste toner collecting device and comprising a pair of position fixing protrusions (51a,53a,55a,57a) at the rear part; a developing roller (52,54,56,58) rotatably mounted to the cartridge body (51,53,55,57); and a terminal (52a,54a,56a,58a) positioned on the cartridge body (51,53,55,57) such that the terminal establishes an electrical connection with an electrical connector (26,27,28,29) provided on the or each inserting groove (22,23,24,25), when the or each developing device is mounted in the image forming apparatus wherein the or each supporting rail (61,62,63,64) further comprises a first position fixing groove (61a,62a,63a,64a), the image forming apparatus further comprises at least one mounting rail (16,17,18,19) having a second position fixing groove (16a,17a,18a,19a), and the or each pair of position fixing protrusions (51a,53a,55a,57a) are adapted to be inserted into the or each first and second position fixing grooves (61a,62a,63a,64a and 16a,17a,18a,19a) when the or each developing device is mounted in the image forming apparatus.