Plasma processing method

The present invention provides a plasma processing method capable of detecting a step conversion having high stability. The plasma processing method, which performs plasma treatment on a sample by a plurality of steps, as a means to solve existing problems: has a first process of supplying inactive gas while stopping the gas supply of a step at one side and a second process of supplying gas of a step at the other side while stopping the supply of the inactive gas of the first process after the first process; detects the gas amount of the step at one side, which remains in a processing chamber in which the sample is processed with plasma, in the first process; detects the gas amount of the step at the other side, which approaches the inside of the processing chamber, in the second process; and converts from the step at one side to the step at the other side based on the gas amount of the step at one side detected in the first process and the gas amount of the step at the other side detected in the second proce