A flexible shaft is inserted through a sheath constituting an insertion portion of an ultrasonic probe connected to an ultrasonic observation apparatus, an ultrasonic transducer driven to rotate, etc., is attached at the distal end portion thereof, and an ultrasonic wave can be transmitted and received through an acoustic window arranged around thereof. By using hydrocarbon-based oil having a kinematic viscosity of 20 mm2/s as an acoustic medium for filling the surrounding of the ultrasonic transducer in the acoustic window, ultrasonic attenuation at high frequencies can be reduced, decrease of the capacity of the acoustic medium due to volatilization can be reduced, and decrease of the capacity of the acoustic medium due to volatilization is reduced.