PURPOSE: To provide a window screen device having a wire tension mechanism which gives no influence to the housing of the window screen, and has excellent running property, weather resistance, beautifulness, and safety, at a low cost.

CONSTITUTION: The wire tension mechanism 2 of an assembling window screen device 1 uses three wires 11, 12, and 13, and in the two wires 11 and 12 of the wires, one side ends are fixed to the upper part of the first fixed member 20, while the other side ends are extended from the upper end of the movable member 30 to the lower end along the movable member 30, they are straddled in the longitudinal directions of both sides of a lower frame 50 by a straddle member 40 attached to the lower end, and fixed to the lower part of the second fixed member 21. The wires 11 and 12 extended from the lower end of the movable member 30 to the second fixed member 21 are covered with a cover 60 formed integrally in the lower frame 50.