A colour image forming apparatus for forming a colour image on a printing medium, the colour image forming apparatus comprising: a main body; a main feeding part main body; a main feeding cassette part provided in the main feeding part main body, which accommodates the printing medium; a main pick-up roller to pick up the printing medium from the main feeding cassette part; a registration roller which aligns a leading edge of the printing medium; an image forming part included in the main body, which forms a colour image on the printing medium, the image forming part including a transfer belt which transfers the printing medium, and a plurality of developing devices (310Y,310M,310C,310K) which are disposed in a relation to the transfer belt and sequentially from a lower part to an upper part of the main body; and a reversing part provided within the main body that serves to reverse a printing side of the printing medium after printing operation is performed on a first surface thereof by the image forming part, a feeding roller part (130, 131, 133) which re-feeds the printing medium, having been transferred through the reversing part, to the registration roller; and a sub-feeding part pivotally coupled to an outside of the main body, which stores the printing medium therein, wherein the reversing part is configured to allow the printing medium to pass through a reversing path of the reversing part provided in the main body and to allow the printing medium to be guided via the feeding roller part to the image forming part so as to perform printing operation on a second surface of the print medium, and wherein the main feeding part main body is configured to accommodate the feeding roller part therein, and the feeding roller part (130, 131, 133) is further configured to separate the printing medium stored in the sub-feeding part and supply the printing medium to an interior of the main body; characterised in that the main feeding part main body is attachable and detachable with respect to the main body. a main body; a main feeding part main body; a main feeding cassette part provided in the main feeding part main body, which accommodates the printing medium; a main pick-up roller to pick up the printing medium from the main feeding cassette part; a registration roller which aligns a leading edge of the printing medium; an image forming part included in the main body, which forms a colour image on the printing medium, the image forming part including a transfer belt which transfers the printing medium, and a plurality of developing devices (310Y,310M,310C,310K) which are disposed in a relation to the transfer belt and sequentially from a lower part to an upper part of the main body; and a reversing part provided within the main body that serves to reverse a printing side of the printing medium after printing operation is performed on a first surface thereof by the image forming part, a feeding roller part (130, 131, 133) which re-feeds the printing medium, having been transferred through the reversing part, to the registration roller; and a sub-feeding part pivotally coupled to an outside of the main body, which stores the printing medium therein, wherein the reversing part is configured to allow the printing medium to pass through a reversing path of the reversing part provided in the main body and to allow the printing medium to be guided via the feeding roller part to the image forming part so as to perform printing operation on a second surface of the print medium, and wherein the main feeding part main body is configured to accommodate the feeding roller part therein, and the feeding roller part (130, 131, 133) is further configured to separate the printing medium stored in the sub-feeding part and supply the printing medium to an interior of the main body; characterised in that the main feeding part main body is attachable and detachable with respect to the main body.