In a spherical single-element piezoceramic transducer for HIFU (High Intensity Focused Ultrasound) treatment, the modes of plate wave propagation within the ceramic were observed in addition to the pure thickness-mode vibration in experiment. These may potentially influence the therapeutic effects. In this study, we analyzed the vibrational behavior of an air-backed single-element spherical PZT transducer in two ways: by reconstruction of the sound source distribution from the measured ultrasound pressure field, and by numerical simulation of vibration based on a Finite Element Method (FEM). In the two-dimensional Fourier analysis performed in the time and space domains, the radial wavenumber spectrum at the driving frequency showed a high peak, second to the highest peak corresponding to the uniform vibration. This corresponds to waves propagating as leaky Lamb waves from the circumferential edge towards the center of the spherical transducer. These waves propagate at a constant angle with the shell surface in water, and form the relatively large lobes on the acoustic axis on the nearside of the geometric focus. The acoustic energy of the nearside lobes may cause additional near-field heating of tissues which should not to be treated.