PURPOSE: To enable the depth of a pitting to be estimated accurately at an arbitrary point non-destructively by obtaining a resistance coefficient of a corrosion product based on the current value of a monitor device and the resistance constant of corrosion reaction of a metal member and the calculating the depth of pitting based on the resistance constants of these and the potential difference.

CONSTITUTION: The flow rate within sample tubes 1, 2, and 3 is maintained to be constant by a flowmeter 18C and a test water within a test water bath 10 is circulated through a piping 18, the sample tube 1, a piping 20, the sample tube 2, a piping 22, the sample tube 3, and a piping 24 by a pump 18A. A replenishment water is replenished from a replenishment water tank 12 to the water bath 10 by a pump 14A to keep residence time to be constant. Anode currents flowing to lead wires 1b, 2b, and 3b of each local corrosion motor are measured for a long of time, a resistance constant K2 of a corrosion product is calculated from the measurement value and a resistance constant K1 of corrosion reaction of a metal member which is obtained by an experiment, and the constants K1 and K2 and the actually measured values of anode current are substituted into a pitting model equation, thus estimating the pitting depth.