To provide a ball screw with a position retaining mechanism which prevents rotation of a nut by stopping rolling even if reaction force acts on a screw shaft and which can mechanically retain axial displacement of a screw shaft in an arbitrary position.

In a ball screw with a position retaining mechanism for securing an axial position of a screw shaft 1 with respect to reaction force to be loaded, a position retaining mechanism 4 is equipped with an annular exciting coil 8, a rod-like protruded member 10 made of a magnetic material fitted into the exciting coil 8 and an elastic member 9 for biasing the protruded member 10 in a prescribed direction, and buried in a position corresponding to a piece member 6 of a nut 2. An opening 7a is formed in a part of a coupling groove 7 of the piece member 6. By imparting a prescribed applied voltage to the exciting coil 8, the protruded member 10 is sucked against the elastic member 9 and driven in the axial direction to be appeared in the opening 7a. By so doing, rolling of a ball 3 is controlled so as to selectively retain an axial position of the screw shaft 1.