A substrate processing apparatus includes a chamber including a process chamber for performing a process on a substrate by a gas introduced thereto and an exhaust chamber for evacuating the gas in the process chamber, a shield member for separating the process chamber from the exhaust chamber provided in at least a part of a neighborhood of a side wall of the chamber, and a hollow relay member penetrating through the shield member for communicating the chamber with a pipe connected to a pressure gauge outside the chamber. The relay member is configured to receive a first gas flowing from the chamber into the relay member. The first gas has a first conductance. The first conductance is greater than a second conductance of a second gas flowing from the exhaust chamber into a gap between the relay member and the side wall of the chamber.