A medium supply apparatus includes a box member and a lid. A medium is stored inside the box member. The box member is inserted into and extracted from a body of an image forming apparatus, and has a length in an extraction/insertion direction that is longer than a length of the body in the extraction/insertion direction. The lid is provided on a back side of the body in the extraction/insertion direction of the box member to cover at least an upper part of a portion of the box member that projects from the body. The lid includes a first member, a second member, and a first lug. The first member is movable in the extraction/insertion direction with respect to the body. The second member is connected to the first member so as to be relatively movable in the extraction/insertion direction with respect to the first member. The second member is moved by being pushed by the box member which is inserted into the body. The first lug is provided on the second member. The first lug is engaged with the box member by a force that presses the box member against the second member, and disengaged from the box member by a force that extracts the box member from the second member.