An ultrasound probe is located in a chamber defined in a housing to be inserted into a body cavity of a subject. A drive shaft is connected to the probe to transmit a rotational force and, for this purpose, is coupled to a drive source via a passage of an insertion section of an ultrasound diagnostic apparatus. A first sealing member is provided for blocking a communication between the chamber and one end of the passage, a second sealing member is provided for sealing the other end of the passage of the guide member, and a bypass communication passage is provided for enabling the passage to communicate with the chamber. An opening/closing valve is provided for opening and closing the communication passage via which an ultrasound transmitting medium is filled in the chamber and the passage of the insertion passage. The communication passage is blocked after the filling operation has been done.