PURPOSE: To simplify a viscosity sensor and to reduce size and cost by arranging a vibration body in a casing and supporting the periphery of one end of this vibration body by a support member so that the other can vibrates.

CONSTITUTION: Plural large openings 3 which penetrate the casing 2 are formed at one end side of the casing 2, the plate type vibration body 5 which extends in the lengthwise direction of the casing 2 is arranged in the casing 2, and its one end is held by the support member 6 arranged at the center part of the casing 2. Further, a circuit board 20 is inserted in the casing 2 adjacently to the member 6, and this circuit board 20 has an oscillation circuit connected to a piezoelectric element 16 and a metallic plate 15 and a detecting circuit connected between the other piezoelectric element and the metallic plate 15. When the oscillation circuit applies an input voltage of specific frequency with a specific amplitude between the metallic plate 15 and element 16, the vibration body 5 vibrates and its vibration amplitude varies in proportion to the viscosity of liquid. The amplitude of the vibration body 5 varies and then an output voltage also varies, so the amplitude of the output voltage is detected to detect the viscosity of the liquid.