To measure a mass change in real time and accurately in a liquid phase surface treatment such as plating, that is accompanied with the mass change of a body to be treated.

A first quartz oscillator 2 with a mass change detection surface that is subjected to the same surface treatment for a body W to be treated and a second quartz oscillator 3 with the same vibration characteristics but a non-activated reference surface for the surface treatment are simultaneously dipped into a chemical liquid L. An amount of change Δf of a frequency that the first quartz oscillator 2 shows along with the advance of surface treatment is obtained as the sum (ΔfM+ΔfL) of an amount of change ΔfM due to mass increase and an amount of change ΔfL due to the fluctuation in liquid phase parameters. On the other hand, the amount of frequency change indicated by the second quartz oscillator 3 is only the amount of change ΔfL. Therefore, by performing information processing for subtracting the latter from the former by an operation processing device 6, only the amount of change ΔfM can be determined accurately.