To provide an operation string constraining device of a rotary window capable of constraining a hanging down operation string securely.

In a rotary window 6 which rotates using a horizontal shaft as a fulcrum and hangs down an operation string opening, closing, and operating a built-in blind from an upper part of a window frame 61, this operation string constraining device 1 consists of a substrate 2 fixed on a part of the window frame 61 of the rotary window 6, a plate 3 which is supported on the substrate 2 so as to open and close freely and forms a cutout section 3 in which the operation string is hooked and fitted, a spring member 4 energizing the plate 3 in the direction in which it is opened from the substrate 2, and a switch 5 constraining the plate 3 at a position where it is closed on the substrate 2.