A magnetic recording/reproducing apparatus, having a driving source; a main chassis on which the driving source is mounted; a sub chassis which is disposed to the main chassis so as to be slid in a front and rear direction, and which is provided with various tape running elements; a head drum device which is mounted on the main chassis; a loading system provided with a supply side pole base driving device and a winding side pole base driving device which loads a tape depending on the moving operation of the sub chassis in the front and rear direction; a running system which runs the tape in a desired direction at a desired speed; a main power transmitting unit which selectively transmits a power of the power source to the loading system or the running system; a cam gear which is rotated by the main power transmitting unit and transmits the power to each mode; a slider which is disposed between the main chassis and the sub chassis so as to be slid in a left and right direction and interlocked with the cam gear; a mode detecting unit which detects each mode according to the tape running; a tape tension regulating unit which applies a proper tension to the tape; an automatic separating unit which automatically blocks a power transmission to the loading system in a particular mode; a connecting unit which permits the power transmission to the loading system in a particular mode; and a connection maintaining unit which maintains the power transmission at the time of loading/unloading the tape or in the early re-connection which transmits again the power to the loading system.