To improve the transmitting and receiving characteristics of piezoelectric micromachined ultrasonic transducers (pMUTs), the thickness of the diaphragm material SiNx of the pMUT was investigated by vibration analysis using the finite element method (FEM). The vibration magnitudes increase 1.7-fold by changing the SiNx thickness from 1.0 to 0.5 µm as determined by the FEM results. A pMUT with a new structure that has a 0.5-µm-thick SiNx layer was fabricated and evaluated. The vibration of the pMUT was evaluated by a laser Doppler vibrometer (LDV), and the vibration magnitude of the fabricated pMUT was threefold that of our previously reported pMUT. Ultrasonic characteristics such as transmitted ultrasonic sound pressure and ultrasonic sensitivity are also evaluated and compared with those of the previous pMUT. The fabricated pMUT shows 11 times larger transmitted ultrasonic sound pressure and 5 times larger ultrasonic sensitivity than those exhibited by the previous pMUT.