An internal pressure explosion-proof system by which an electric motor or the like is not rendered operative until after gas in an airtight vessel is replaced with certainty by protective gas. The internal pressure explosion-proof system includes a gas flow detector provided in an exhaust pipe for detecting an amount of protective gas exhausted from an airtight vessel in which an electric appliance which may produce a spark or fire is accommodated, and a controller for enabling operation of the electric appliance when a flow amount detected by the gas flow rate detector when protective gas is fed into the airtight vessel reaches a predetermined level and the internal pressure of the airtight vessel exceeds another predetermined level.