A deck mechanism of a magnetic recording/reproducing apparatus which is miniaturized and lightweight includes a main deck (1) where a head drum (6) is installed, a sub-deck (2) which moves with respect to the main deck (1) and where a reel table (20, 24) is installed, and a slide member between the main deck (1) and the sub-deck (2) and which moves by a cam gear (4) and selectively operates a pole base (17, 18), brake (47, 54, 55) and pinch roller (73).