A sensing instrument using a piezoelectric resonator changes natural frequency when coming into contact with a liquid to adsorb a substance to be sensed in the liquid. The sensing instrument includes an oscillator circuit supplying a piezoelectric resonator with oscillation driving power for oscillating the piezoelectric resonator so as to set a driving current to 0.3 mA or less. A measuring unit measures a concentration of the substance to be sensed based on an oscillation output of the oscillator circuit. Self-heating of the piezoelectric resonator is suppressed. Consequently, a change of the oscillation frequency of the piezoelectric oscillator is suppressed, which as a result enables accurate detection of a variation of the frequency due to the adsorption of the substance to be sensed.