To continue measurement of a seawater state, while exchanging the power source or the memory.

When a switch 40 is turned off in exchanging an external device 12 by a diver, supply of electric power from a battery 32 of the external device 12 is stopped, and each element of a measuring instrument 10 cannot operate in leaving the state. A power source changeover part 60 supplies electric power obtained from a backup power source 62 to each element of the measuring instrument 10. Thus, even if the supply of the power from the battery 32 of the external device 12 is stopped, the power will be supplied from the backup power source 62 and each element of the measuring instrument 10 can operate. Even if the external device 12 is in the midst of being replaced, power is obtained from the backup power source 62, since seawater state information can be acquired, and the seawater state information obtained to operate each element of the measuring instrument 10 to be measured is stored to RAM 56.