PURPOSE: To surely insert a tape cassette especially of miniature type to the prescribed mounting position of a cassette holder, in a cassette loading device where a standard tape cassette or miniature tape cassette is inserted in the cassette holder and moved to a prescribed loading position respectively.

CONSTITUTION: On a opening part 111e of a door 111 of standard tape cassette, guide parts 111e1, 111e2 guiding both sides of miniature tape cassette 11 are formed, and similarly guide parts 112e1, 112e2 are formed also on a door 112 for miniature tape cassette. A U-shaped arm 70 is suspended on a ceiling board of the cassette holder, and arm parts 70b, 70c of this U-shaped arm 70 are also provided with a position restricting function for both sides of the tip part of miniature tape cassette 11. When the miniature tape cassette 11 is inserted, the miniature tape cassette 11 can be surely inserted to the prescribed loading position of the cassette holder by the above-mentioned guide members.