To provide a substrate processor which can more correctly detect and calibrate a gas flow rate compared to a conventional case, and can perform a precise processing with the correct gas flow rate without increasing installation space and manufacturing cost.

Branch piping 18 is disposed which is branched from the upstream side of opening/closing valves 13d and 14d arranged near the entrance of the processing chamber 11 of a gas supply system supplying raw gas and is connected to exhaust piping 17. A gas flow rate detection mechanism 19 is inserted into branch piping 18, and opening/closing valves 13h and 14h are disposed for switching a passage to a processing chamber 11-side and branch piping 18. The gas flow rate detection mechanism 19 makes gas flow into a resistor, measures pressure at both ends, and detects the gas flow rate from a pressure difference. Mass flow controllers 13a and 14a are detected or proofread by a detected value.