In a cassette loading apparatus used for carrying a tape cassette from an unloading position to a loading position, cutouts are made in side plates to be continuous to L-shaped guide slots made therein so that the cassette holder unit having outwardly projecting pins can be inserted from the top when assembling the same with the side plates. After the cassette holder unit is assembled, a top plate having projecting plates may be attached to the side plates so that the cutouts are filled with the projecting plates. The projecting plates have rounded corners for smooth movement of the projecting pins along the L-shaped guide slots.