Method for driving member and processing apparatus

A processing apparatus comprising:

• a member provided in a processing chamber;
• a driving unit configured to drive the member;
• and
• a control unit,
• wherein the control unit is configured to control:
• irradiating to the member measurement light having a wavelength that penetrates the member;
• detecting intensity distribution of reflected light based on reflected light from an upper surface of the member and reflected light from a bottom surface of the member;
• calculating an optical path difference by applying Fourier transform to a spectrum representing the intensity distribution;
• determining a driving amount of the member based on the optical path difference;
• and
• driving the member based on the determined driving amount.