PURPOSE: To facilitate the attaching work and at the same time to increase the effective length of a piezoelectric bimorph by attaching a ring-shaped holding plate to the end face of a rotary drum at the position of the inner circumferentail side of a piezoelectric oscillator and at the same time holding and fixing the piezoelectric oscillator by means of a projection provided to said holding plate.

CONSTITUTION: The outer diameter d1 of a ring-shaped holding plate 18 having elasticity is smaller than the inner diameter d2 of a piezoelectric bimorph 6. The plate 18 fixes the bimorph 6 to a rotary drum 1. The projected bars 18a and 18b are provided as opposed to each other to the plate 18 with a 180° space secured between them. These projected bars contain small projections 19-1 and 19-2 to hold the bimorph 6. The screws 9 are attached inside further than the inner circumference of the bimorph 6, and therefore the outer diameter of the bimorph 6 is increased up to the limit of the outer diameter of the drum 1. Thus it is possible to increase the radius, i.e., the effective length l of the bimorph 6.