An electrode rod 71 serving as a first electrode is provided to be in contact with a flow path member and a processing liquid of a processing liquid supply path 2. If the processing liquid is flown in the processing liquid supply path 2, static electricity is generated by friction so that the processing liquid and the flow path member is electrically charged. By allowing the electrode rod 71 to be closely contacted with the flow path member, the amount of electric charges corresponding to the sum of the charge amounts of the processing liquid and the flow path member is measured as a surface potential of the first electrode by a surface potential measuring unit 77. The measured surface potential is displayed on a display unit 201.
