Plasma processing method

A plasma processing method for plasma-etching an etching target film using a protective film formed on a mask, the plasma processing method comprising:

• a step of forming the protective film on the mask using plasma generated by a mixed gas of a silicon element-containing gas, an oxygen element-containing gas, a nitrogen element-containing gas, and a hydrogen element-containing gas, wherein
• in the step, the protective film is formed while supplying radio frequency power to a sample on which the etching target film is formed,
• radio frequency power for generating the plasma and the radio frequency power supplied to the sample are pulse-modulated,
• a duty ratio of the radio frequency power supplied to the sample is smaller than a duty ratio of the radio frequency power for generating the plasma, and
• an on period of the radio frequency power supplied to the sample is included in an on period of the radio frequency power for generating the plasma.