To attain locking without using a screw locking agent by providing a fixing screw disposed in a fixed frame, with a locking elastic part generating force in the axial direction, and a fastener for supporting the movable end of the elastic part.

A fixing screw 16 in a fixed frame 15 is provided with a spring 2 and a nut 1 fixed being threadly engaged with the fixing screw. When the fixing screw 16 is fastened, the end of a case 11c is fixed to a panel through a threaded hole 15a, and the fixing screw 16 receives axial force through the spring 2 so as to obtain locking function of the fixing screw 16. A human- initiated error can therefore be prevented without using a screw locking agent so as to obtain a instrument structure of high reliability against vibration and impact.