An internal pressure adjustment system 3 includes a gas supply line 100 that supplies incombustible gas to a housing space 51 of a robot 2, an exhaust line 300 that exhausts gas from the housing space 51, and an on-off valve 410 of a gas pressure driven type that switches between opening and closing of the exhaust line 300 in accordance with the gas supply pressure so that the exhaust line 300 is opened in response to an increase in the gas supply pressure and the exhaust line 300 is closed in response to a decrease in the gas supply pressure.
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