There is provided a technique that includes: a mounting table configured to be capable of mounting a substrate on the mounting table; a heater located above the mounting table and configured to radiate heat rays toward the substrate mounted on the mounting table to heat the substrate; a process container disposed below the heater and configured to accommodate the mounting table, wherein at least a portion of the process container, which is located adjacent to the heater, is made of opaque quartz; and a sealer configured such that transmission of heat rays reflected from the substrate to the sealer is suppressed by the at least a portion of the process container and airtightness between the process container and the heater is maintained by a seal.