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 and an acoustic velocity of the propagation medium portion, and a density and a sound velocity of a fluid that fills 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.
An ultrasonic receiver according to the present invention includes a first horn having a first large opening and a first small opening which are an incidence end and an outgoing end of an ultrasonic wave; a second horn having a second small opening and a second large opening which are an incidence end and an outgoing end of the ultrasonic wave, the first and second horns being located such that the first and second small openings face each other, and such that propagation directions of the ultrasonic wave in the first and second horns match each other; and at least one ultrasonic receiver main body provided between the first and second small openings, the ultrasonic receiver including a surface parallel to the propagation direction and detecting the ultrasonic wave which has propagated in the first horn and then is incident on the parallel surface. A cross-sectional area vertical to the propagation direction in the first horn decreases from the first large opening toward the first small opening; and a cross-sectional area vertical to the propagation direction in the second horn increases from the second small opening toward the second large opening.
