Described herein is a technique capable of reducing a thermal damage to a furnace opening structure when processing a substrate at a high temperature. According to one aspect thereof, there is provided a substrate processing apparatus including: a reaction tube provided with a furnace opening; heaters provided respectively in a plurality of zones arranged along a tube axis direction; temperature sensors respectively corresponding to the zones; a temperature controller configured to control electric power based on temperature data obtained by the temperature sensors, wherein the temperature controller is configured to, when the substrates are subject to a heat treatment process by the heaters, control the electric power supplied to the heaters such that temperatures of upper heaters about as high as the substrates reach predetermined temperatures, and that a temperature gradient is formed in lower zones lower than the substrates such that a temperature decreases toward the furnace opening.