Abstract: An ultrasonic diagnostic probe comprising an inflatable balloon member attached to the free end of a tubular support member. An array of elongated piezoelectric elements is arranged in the balloon member. Space ensuring side walls are arranged in the balloon member between the balloon member and the array of piezoelectric elements for ensuring a space in which the array of piezoelectric elements can rotate irrespective of the shape balloon member. The balloon member can be inflated by introducing liquid therein. The balloon member is in a deflated condition when the probe is inserted in the esophagus, and can be inflated after the probe is inserted in the esophagus to allow the array of piezoelectric elements to be rotated. The space ensuring side walls are correspondingly deformed.