To establish a master key system of high safety with one lock.

This disk tumbler lock is provided with an outer cylinder 2 formed with grooves 7 allowing the advance of upper ends and lower ends of a plurality of disk tumblers 6A, 6B and 6C, and an inner cylinder 3 inserted in the outer cylinder 2 to so as to be turnable enclose the disk tumblers 6A, 6B and 6C lined up in an internal hollow part 9 opened on the upper and lower sides. Each disk tumbler is formed with a projecting part 17 at the one-side edge of an opening of a key hole 14 formed in the center so as to be engaged with one probe groove 13 or 23 of a key selected out of a master key Q1 and a slave key Q2 according to a purpose, and a step 18 oppositely formed at the other side edge of the opening of the key hole 14 so as to be engaged with the other probe groove 13 or 23 of the key. The projecting parts and steps are slidably brought into close contact as pairs in a vertically shifted state, and energized by springs 12 to repel each other in a vertical direction.