There is provided a method of processing a target object to be processed including a porous film and a mask. The method include supplying a first gas into a processing chamber of a plasma processing apparatus in which the target object including the porous film is accommodated, and generating a plasma of a second gas in the processing chamber to remove the mask. The first gas is a processing gas having a saturated vapor pressure of less than or equal to 133.3 Pa at a temperature of a stage on which the target object is mounted in the processing chamber, or includes the processing gas. In the step of supplying the first gas, no plasma is generated, and a partial pressure of the processing gas supplied into the processing chamber is greater than or equal to 20% of the saturated vapor pressure.