PURPOSE: To obtain a strong self lock mechanism by providing plural pairs of cylindrical rollers which are arranged in parallel to the center axis line between a rotary shaft and an outer ring and providing a spring means for always energizing each cylindrical roller, and an energizing means for resisting the above-described urging force when necessary.

CONSTITUTION: The inner wall surface of an outer ring 14 which is concentric to a rotary shaft 10 has a symmetrical center line 18 passing through a center axis line 12 and four curved surfaces 16 having the radius R larger than the radius (r) of the shaft 10. While, the center parts of two cylindrical rollers 24a and 24b having a conical edge part 26 is crossing-penetrated by a rod member 20 inserted into the recessed part 22 of the outer ring 14, and wet close to the direction of the symmetrical line of the inner surface 16 by a coil spring 28 energized at all times and nipped between the inner surface 16 and the shaft 10, and one cylindrical roller is attracted by the revolution of the shaft 10, and the control effect is improved, and the revolution of the shaft 10 is stopped. Further, the projecting members 32 and 34 having a tapered part 30 fitted with the inclination of the conical edge part 26 are pressed onto the cylindrical rollers 24a and 24b, and separated from the symmetrical line 18, and lock is released. Therefore, a compact self lock mechanism having a strong stop holding force and high reliability can be obtained.