Spurious response resulting from a high-order vibration mode that occurs when the cell shape of a capacitive micro-machined ultrasonic transducer is anisotropic is reduced. Assuming that a ratio between a long direction (l) and a short direction (w) of a diaphragm forming a capacitive micro-machined ultrasonic transducer is a representative aspect ratio (l/w), the representative aspect ratio is set to a value at which a dip of 6 dB or greater would not be formed within a transmit and receive bandwidth of a probe. Alternatively, the representative aspect ratio is so set that there would be six or more vibration modes for which the value obtained by dividing the frequency of a vibration mode having an odd number of anti-nodes by a fundamental mode frequency would be 2 or less.
