PURPOSE: To make it possible to measure the pH of an electroplating bath with good accuracy within a short time, by a method wherein the absorbancy of the pH indicator to be added to the electroplating bath is measured at the isosbestic point thereof and the error based on the change in the flow amount of an indicator solution and the deterioration thereof with the elapse of time is corrected by using the measured value.

CONSTITUTION: A pH-indicator having a wavelength not changed in absorbancy by pH(called as an isosbestic point), for example, m-cresol purple has a wavelength changed by pH of 525nm at an isosbestic point of 478nm. This indicator solution is sent from a storage tank 3 through a valve 5 and mixed with an electroplating bath 1 in a mixer 6 while the liquid mixture is sent to an absorbancy measuring cell 7. Light is irradiated from a light source 8 while pervious light and reflected light of a half-mirror 9 are respectively passed through interference filters 10, 11, detectors 12, 13 and electric signal converters 13, 14 to calculate the ratio of the absorbancy A525-B525 in an indicator mixed case A and the absorbancy A478- B478 in an indicator non-mixed case B while this ratio is substituted for a preliminarily calculated pH conversion formula to calculate pH with good accuracy. The manipulations of pumps 2, 4 and a valve 5, operation and recording are automatically performed by a treatment apparatus 16.