The magnetic recording/reproducing apparatus for recording onto or reproducing from a magnetic tape housed in a cassette by using a rotary magnetic head has tape drawing-out means (6, 8, 9, 55) for winding a portion of the tape (53) between the two reels in the cassette around the rotary magnetic head (2) by a predetermined angle. The magnetic recording/reproducing apparatus has further tape driving means (3, 57) for feeding the magnetic tape at a constant speed, reel driving means (24) for driving either one of the two reels so as to take up the magnetic tape in the reel driven by the reel driving means, and braking means (18,19, 20) for braking the rotation of the reels. Each of the above described means is selectively actuated by a ring-shaped cam operating member (64) provided in the outer peripheral surface or the inner peripheral surface thereof with cam surfaces each for effecting the operation of the respective means in response to the mode of operation of the magnetic recording/reproducing apparatus. The selective operation is specifically determined by the rotational position of the ring-shaped cam operating member, and the rotational position is varied in response to the mode of operation. Detecting means (62, 72) for detecting the rotational positions is provided in the ring-shaped cam operating member.