To appropriately control the concentration of a stabilizer containing sulfur that is not affected by other constituents or the like by adding the stabilizer containing sulfur up to a deposition stop potential in an electroless nickel bath and measuring the amount of increase in the concentration.

The concentration of a stabilizer containing sulfur of thiosulfate or the like contained in an electroless nickel bath is measured by measuring a deposition stop potential Estop that is a potential in a state that deposition stops when the stabilizer containing sulfur is further added. When the stabilizer containing sulfur is added to an electroless nickel bath S0 without any stabilizer containing sulfur and the concentration reaches Sstop, deposition stops and potential being measured by a specific method changes from a deposition potential Eplate to a deposition stop potential Estop. The amount of addition (amount of increase in concentration) at this time is set to V0. Similarly, for an electroless nickel bath to be inspected where the concentration of the stabilizer containing sulfur is unknown S, the amount of increase in concentration V until the deposition stops is obtained. The unknown concentration S can be obtained as V0-V.