To enable the operation for inserting or taking out a cartridge smoothly with the light force by allowing the attaching/detaching operation of the cartridge in the manner of sliding it with the motor-driven system by a driving source.

Since the magnetic recording/reproducing device is constituted so that the driving source 13 is driven or stopped by making a 1st slide member 5 to slide and operating detection members 16a, 16b, 16c with the slide of a working member 14 in the attaching/detaching operation of the cartridge 4 by driving the driving source 13, the loading or ejecting operation for attaching/ detaching the cartridge 4 is executed by the motor-driven system, then the cartridge 4 is easily attached or detached by the extremely light force.