PURPOSE: To obtain an inexpensive rotation sensor the structure of which is simple and the assembly is easy.

CONSTITUTION: This sensor comprises a doughnut type cylindrical housing 12 which is fitted on a ratatable target object, having a cavity interposed between an outer wall 14 and an inner wall 16, a fluid 18 disposed in this cavity, a piezoelectric transducer 20 fitted so as to extend in the cavity and fluid, and a capacitive diaphragm or a strain gauge. The rotation of the target object gives relative rotation movements between the housing 12 and the fluid 18, and these relative movements deflect the piezoelectric transducer 20, etc., in proportion to the rotation of the target object.