To provide a window locking device to be used as a crime-prevention device for double sliding window, capable of surely preventing a user from forgetting to lock a double sliding window after unlocking it by giving an alarm sound to announce non-locking when the double sliding window is unlocked.

The window locking device includes a locking device body which is attached to a window frame of one window of the double sliding window, and includes a protruding and retreating dead bolt; and a receiving seat which is attached to the glass surface of the other window of the double sliding window, and performs locking of the double sliding window by being engaged with the dead bolt protruded from the locking device body. When an operation part is operated with the dead bolt being engaged with the receiving seat, an unlocking mode for retreating the dead bolt to unlock the double sliding window and giving an alarm sound from the sound generation part is established. In the unlocking mode, the alarm sound is automatically given at fixed intervals. The user can recognize that the window is in the unlocked state even if it is covered with a curtain or the like by giving the alarm sound.

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