We propose a simple calibration method for a hydrophone using a small tank. Even a tank equivalent in size to the wavelength of a projected sound can be used for the measurement of hydrophone sensitivity at the fundamental resonant frequency of the tank. Under such circumstances, the sound pressure level in the tank is high and nearly constant within the proposed stable area. A stable exposure level can be observed as a 0.3-m sphere at the center of the tank up to 3 dB, which is sufficiently large compared to the size of a piezoelectric sensor. A relatively constant sound field area appears only at the resonant frequency of the tank, which can be used for a quantitative measurement of hydrophone sensitivity. When a sound is projected at a frequency higher than the fundamental resonant frequency, the sound pressure level changes in a complex manner, owing to constructive or destructive phase interference. A received sound pressure level at a frequency lower than the fundamental resonant frequency exhibits a quick reduction. In either case, the sound pressure level is location-sensitive, which makes the calibration of a hydrophone in a small tank difficult.