To surely transfer a rotational force of a square core by bringing a hub hole of a latch hub into contact with a shaft core fitted and inserted into the hub hole, with no space between them, by means of a simple constitution, and to make the square core excellent in durability and economy.

This square core is formed of two plate bodies 11 and 12 which are split along a central axis line. In the two plate bodies 11 and 12, inclined planes 112 and 122 with angles equal to each other are each formed in an opposed state along an edge on the side of a mating face. When the two plate bodies 11 and 12 are superposed on each other so as to be inserted into square holes 51 and 72 of operating members 5 and 7, a tip of a fastening screw 8, which is screwed into the operating member 7, gets into a V-shaped cross-section groove streak 2 which is formed of the respective inclined planes 112 and 122, and the two plate bodies 11 and 12 are mutually pressed open so that the operating members 5 and 7 and the hub 6 can be fixed.