To provide a looseness preventing washer which is effective, easy and inexpensive to prevent the looseness of a screw with the washer 1 put therein when design is needed to reduce a difference between the inner diameter and the outer diameter of the washer 1, and which is easy and inexpensive to automatically assemble while eliminating the need for machining a set screw with a hexagon socket being used for preventing the looseness of the screw between a piston 5 of a standardized fluid pressure cylinder and a piston connection rod 6, in particular.

The screw looseness preventing washer 1 comprises a ring disc 2 and protruded and recessed rings (3, 4) to be mounted between the piston 5 and the piston connection rod 6 while fastening a threaded portion 7 with specified torque to be put in close contact with the protruded and recessed rings (3, 4). Spring force generated at this time is applied to actualize hard looseness. It is acceptable to use two units with the combination of 2, 3 or 2, 4.