In a concentration sensor using a piezoelectric piece, for instance, a quartz piece in which the natural frequency is varied by adsorption of a sensing target, the object of the present invention is to make the concentration sensor applicable to various fluids to be measured different in viscosity while restricting consumption of the oscillation energy due to interelectrode capacitance, and to be able to cope with plural types of concentration sensors while using a common oscillation circuit. As a concrete means for solving the problem, an inductor is connected in parallel to the piezoelectric piece, and its inductance value is determined based on the relation between inductance values and oscillation loop gains in a gap between the inductance value Lmax canceling the interelectrode capacitance in a resonance frequency of the piezoelectric piece and the inductance value Lmin evaluated as the minimum value among the inductance values with which the piezoelectric piece can oscillate in a state of being immersed in a measuring fluid.