To provide a screw shaft whirl-stop structure of a screw jack having high rigidity and capable of exhibiting strong and excellent whirl-stop function to an impact load, and allowed to be adopted in a ball screw jack using the ball screw.

The screw jack 1 is provided with a main body case 2, a female screw body 11 rotatably provided on the main body case 2 to be driven to rotate by transmitting means 3, 4, a screw shaft 12 screw-engaged with the female screw body 11, to penetrate therethrough and reciprocated in the axial direction, and a protective tube 16 fixed to the main body case 2 to cover and protect the screw shaft 12 extended from the main body case 2. In the jack 1, the protective tube 16 is formed out of a square pipe, tie rods 19 are disposed along the inner four corners, a whirl-stop body 22 fixed to the rear end part of the screw shaft 12 is formed like a quadrangle fitted to the inner surface of the protective tube, and four corners of the whirl-stop body 22 are provided with cutout parts 25 engaged with the tie rods 19.