Gear transmission device and image forming apparatus including the same

A power transmission device, comprising:

• a first frame;
• a drive motor including a rotary shaft attached to a first side of the first frame, the rotary shaft including first external teeth on an outer circumferential surface of the rotary shaft;
• a first driven rotatable gear near the rotary shaft, the first driven rotatable gear including an annular concave portion including an internal teeth portion on an inner circumferential surface of the annular concave portion;
• a second driven rotatable gear near the rotary shaft, the second driven rotatable gear including second external teeth on an outer circumferential surface thereof;
• wherein when the annular concave portion faces the first frame, the first external teeth of the rotary shaft are configured to be inserted into the annular concave portion so as to engage directly the first external teeth and the internal teeth, and the second external teeth are configured to engage the first external teeth which are not in mesh with the first driven rotatable gear.