Abstract: A tumbler lock comprising housing pockets in an outer housing and plug pockets in a cylindrical plug rotatably mounted in the housing holding slidably outer and inner pins respectively, auxiliary pockets in the housing corresponding to the plug pockets at a certain rotated position circumferentially spaced from the housing pockets holding therein auxiliary pins, and change pins which may be interpositioned between the inner, outer and auxiliary pins and interchanged between the respective pockets so that the change pins may be freely shifted to reset the back to be operated by different keys.