Conventional way of underwater inspecting work of rubble mound for breakwaters is necessry to do improvement because of severe construction condition, and decrease and old aged divers is on the increase.
　To cope with these problems, small-size measuring instrument which measures underwater distance and water depth is developed and measuring performances and accuracy has been tested through laboratory test and field test.
　This system consist of two instruments and can measure the distance between two points, water depth and the difference of water depth at two points.
　The pressure sensor and ultrasonic sensor of high accuracy are introduced, and measured values are shown on the display of the instrument and are transmitted to
a computer.
　The length is 1.4 - 1.7m long and weight is 5 kg underwater for the underwater portability.
　The accuracy is tested through laboratory test, and performance is tested through field test.
　The manuevability of this instrument is also investigated by the opinion of the divers who carried out the field test. The result is the instrument can be used at the practical works.