Abstract: A magnet card type lock includes an outer case, a slider having a key hole slit formed therein, an engagement member firmly secured to the outer case, a plurality of tumbler holes formed in conformity with a predetermined pattern of arrangement, a plurality of magnet tumblers bridged between the case and the slider, a projection integrated with the slider to be engaged with a locking mechanism and a magnet card adapted to be inserted into the key hole slit of the slider to serve as a fellow key. To prepare a specific key difference among a number of magnet card type locks, a predetermined number of tumbler holes are selected from the tumbler holes which extend at a substantially right angle relative to the slider and the outer case. The magnet card is formed with a pluraltiy of apertures some of which are immovably filled with permanent magnets corresponding to the selected number of tumbler holes.