PURPOSE: To simply connect an internal window frame and an external window frame and securely insulate the heat transfer between the window frames.

CONSTITUTION: Coupling holes 8, 9 are formed in an external window frame 2 and an internal window frame 3. A coupling device 10 is constituted of a heat-insulative coupling member 11 having a fitting part 16 inserted in the coupling hole 9 and a screw 18 engaged with a female screw of the coupling member 11 through the coupling hole 9. The fitting part 16 is arranged at a position where it can be fitted and the screw 18 is rotated through the coupling hole. In this way, the fitting part 16 is transferred to the coupling hole 8 side according to the rotation of the screw 18 and engaged in the coupling hole 8 and the external window frame 2 is fixed with the coupling member 11 and also the internal window frame 3 is fixed to the coupling member 11 with the screw. And hence, respective window frames 2, 3 are coupled to each other through the coupling member 11.