To prevent the overrun of a moving body by setting a magnetic belt of a magnetic screw rod to nonmagnetization by a specified length in one end part or both end parts of the rod and by preventing the rotation movement of the magnetic screw rod from being transmitted to respective magnetic sleeves, when the respective magnetic sleeves reach the nonmagnetized zone.

In a scanning optical device 14' using magnetic screw rods 1, 1', a first and a second slider unit 4, 5 reciprocate linearly and when a control anithmetic unit falls into an out-of control state, the first and the second magnetic sleeves 2, 2', 3, 3' reach non-magnetized zones 71, 71' so as to forcedly stop it. In such an out-of control state as the first slider unit 4 advances in the right direction, when the first magnetic sleeve 2, 2' reach the non magnetic zones 71, 71', the drive force from the magnetic screw rods 1, 1' is not transmitted thereto so as to stop. This constitution can prevent the first slider unit 4 from colliding with driven pulleys 7, 7' and other components to cause serious damage.