An objective of the present invention is to accurately measure an active oxygen surface effect volume in a surface process using active oxygen. This active oxygen volume measurement device comprises: a first sensor further comprising a first oscillator which is positioned near an object to be processed, and which outputs a mass change in the first oscillator based on the active oxygen effect volume as a first frequency change quantity; a second sensor further comprising a second oscillator which is positioned near the object to be processed, and which outputs a mass change in the second oscillator based on a disturbance caused by a change in temperature, humidity, or pressure, as a second frequency change quantity; and a measurement unit which is configured to compute the difference between first frequency change quantity and the second frequency change quantity, and to identify the difference as a frequency change quantity by the actual active oxygen effect volume.
