A magnetic recording/reproducing apparatus includes a main chassis on which a head drum is mounted and a cassette holder to support a tape cassette. The cassette holder is mounted slidably on the main chassis. A pivoting lever is mounted pivotably on the main chassis and if operated in association with the cassette holder. A sensing switch provides information on insertion or ejection of the tape cassette, wherein the sensing switch is connected or separated according to the position on which the pivoting lever is pivoted.