We propose a micro ultrasonic wave sensor with a Helmholtz resonator (HR) which enhances its sensitivity. The sensor that consists of a cantilever and an HR demonstrated the 23.8 dB sensitivity enhancement against 5.19 kHz ultrasonic wave than that of the cantilever alone. This means that, by adding the HR, the sensitivity becomes 15.5 times higher. The sensor size against the signal frequency over 20 kHz is able to be fabricated smaller than 2 mm×2 mm×1 mm. We also used a frequency-variable HR to measure the dependence of the sensitivity enhancement on the difference between the resonance frequency of the HR and that of the cantilever. As a result, we confirmed that the enhancement was decreased when the frequency difference was sufficiently large. The sensitivity enhancement was halved when the resonant frequency of the HR became 92.6 or 108.9% of that of the cantilever.