A substrate processing apparatus includes: a reaction tube configured to accommodate a substrate holder holding a plurality of substrates and process a substrate held on the substrate holder; a heating unit installed outside the reaction tube and configured to heat an inside of the reaction tube; a protection tube installed to extend in a vertical direction in contact with an outer wall of the reaction tube; an insulating tube disposed inside the protection tube and having through-holes extending in a vertical direction; a thermocouple having a thermocouple junction provided at an upper end thereof, and thermocouple wires joined at the thermocouple junction and inserted into the through-holes of the insulating tube; a gas supply unit configured to supply a gas, for processing a substrate accommodated in the reaction tube, into the reaction tube; and an exhaust unit configured to exhaust a gas from the reaction tube.