PURPOSE: To obtain mold material of low specific gravity which is used as mold material for making the underwater weight of the transmitter-receiver main body of an underwater ultrasonic transmitter-receiver zero, to establish the manufacturing method and to obtain the underwater ultrasonic wave transmitter-receiver molded by this material of low specific gravity.

CONSTITUTION: By the manufacturing method that a very small hollow ball 12 is blended in rubber which is below rubber hardness 10° or in gel material by volume ratio 40 to 60%, mold material 11 of low specific gravity is formed by uniformly dispersing very small hollow balls 12 in low hardness rubber 13 composed of rubber or gel material. Further, this mold material of low specific gravity is applied, a transmitter-receiver main body is molded and an under water ultrasonic transmitter-receiver which is excellent in an accoustic characteristic is obtained.