PURPOSE: To enable accurate position measurement irrespective of changes in temperature and in salt concentration.

CONSTITUTION: An echo ranging device comprises a plurality of reference sound sources 14a-d disposed at predetermined intervals on a predetermined portion of the sea bottom 16, a receiver 13 disposed at a point 12 of measurement to receive signals from the reference sound source 14a-d, and an arithmetic unit to which signals of the receiver 13 are input. The arithmetic unit compares predetermined positions of the reference sound sources relative to one another with those input from the receiver 13 to find the speed of sound in a sea area for measurement, thus measuring the positions according to the speed of sound.