To make many different keys of a cylinder lock.

A first key way 4 makes internal contact with four sides of a flat rectangle 1, and line-symmetric with respect to both lines which perpendicularly bisect a long side and a short side of the rectangle, respectively. A second key way 8 is obtained by outward protruding a part of an external line of the first key way and cutting the other part of the external line, whereby the second key way is rendered point-symmetric with respect to an intersection point of orthogonal lines of the flat rectangle. A third key way 9 is obtained by longitudinally or laterally inverting the second key way. The cross-sectional shape of a blank of a master key is defined by a fourth key way 11 which includes the first to third key ways 4, 8, and 9, and commonly makes internal contact with the first to third key ways.