A mode-change mechanism for a tape recorder has a driving bar (50) mounted for movement between a record/playback mode position and a stop mode position (S) and a partially toothless gear (38) has a cam (65) thereon engagable by a cam follower (54) on the driving bar (50) to move the driving bar (50) between its mode selecting positions in response to turning of the paritally toothless gear (38) between immobile positions thereof in which opposed first and second toothless portions (66, 67) on the outer periphery respectively face a driving gear (40) intermittently to rotate the partially toothless gear (38). A lock lever (71) is selectively engagable with one of a pair of stop elements (68, 69) on the partially toothless gear (38) to halt rotation thereof in a respective one of the immobile positions thereof, and a spring (53) urges the driving bar (50) toward one of its mode selecting positions (S), with the cam (65), the toothless portions (66, 67) and the stop elements (68, 69) on the partially toothless gear (38) being angularly positioned with respect to each other so that, when the lock lever (71) is disengaged from one of the stop elements (68, 69), the force of the spring (53) on the driving bar (50) is effective, through the action of the cam follower (54) on the cam (65), to initiate turning of the partially toothless gear (38) from its prior immobile position into meshing engagement with the driving gear (40). The toothless portion (66) which faces the driving gear in the record/playback mode is longer (ϑ₁) than the length (ϑ₂) of the toothless portion (67) which faces the driving gear (40) in the stop mode so that substantially instantaneous change from the record/playback mode to the stop mode can be achieved upon disengagement of the locking lever (71). Further, the partially toothless gear (38) for controlling the selection of the record/playback mode and the stop mode, a similar partially toothless gear (39) for controlling the selection of forward and reverse tape running modes, and a group of driving gears for selectively driving reel bases (7, 8) are arranged in surrounding relation to an intermediate driving gear (45) which intermittently rotates the partially toothless gears (38, 39) so that all of the gears may be accommodated in a substantially small space.