The present invention is aimed to perform precise monitoring of the processed amount by which a workpiece is processed, and includes a measurement unit that measures a concentration or a partial pressure of a reaction product generated while the workpiece is being processed, and an operation unit that calculates the processed amount of the workpiece using an output value of the measurement unit. The measurement unit includes: a laser light source that irradiates target gas containing the reaction product with a laser beam; a photodetector that detects a laser beam having passed through the target gas; and a signal processing unit that calculates the concentration or the partial pressure of the reaction product based on a detection signal of the photodetector. The operation unit includes a time integration unit; a relationship data storage unit; and a processed amount calculation unit.