There are provided an ultrasonic transducer that performs transmission and/or reception of an ultrasonic wave and a propagation medium portion that forms a propagation path of the ultrasonic wave. A density ρ1 and an acoustic velocity C1 of the propagation medium portion and a density ρ2 and a sound velocity C2 of a fluid that stuffs a circumjacent space are appropriately set, and a propagation loss of the ultrasonic wave into the fluid is reduced to almost zero by refracting the ultrasonic wave at an appropriate angle.