To provide a locking device capable of preventing illegal breaking-in by a thief by providing crime prevention elements in a double sliding sash window comprehensively.

This locking device for the double sliding sash window is composed of a main lock and a sub-lock arranged on a meeting stile of an internal paper screen, a slide plate provided internally on the meeting stile of the internal paper screen, and a lock receiver provided on a meeting stile of an external paper screen. The main lock has a rotary cylinder structure provided with a knob for operation, a locking hook is formed at a tip of a cylinder, and a cut-out cam is formed at a predetermined position close to its tip. The sub-lock has a rotary cylinder structure provided with a knob for operation, a locking hook is formed in a tip part of a cylinder, and a locking channel is provided at a predetermined position close to its tip. The slide plate has a locking hole in which the locking hook formed in the sub-lock is loosely inserted in an upper part.