A safety device (50, 50A), comprising: an airbag supplied with water vapor to be inflated and deployed; an ignition portion configured to ignite a mixed fluid of hydrogen and oxygen to generate water vapor to be supplied to the airbag; and a first flow path configured to guide hydrogen to the ignition portion from a first container filled with a volume of hydrogen in a pressurized state, the volume of the hydrogen in the first container exceeding an appropriate volume for inflation and deployment of the airbag, characterized by further comprising: a first opening and closing portion disposed in the first flow path and switchable between a closed state where a flow of hydrogen from the first container is shut off and an opened state where the flow of hydrogen from the first container is opened; an introduction section (6, 6A) disposed at a position downstream of the first opening and closing portion in the first flow path, the introduction section (6, 6A) introducing oxygen supplied from an oxygen supply source into the first flow path and thereby mixes the oxygen with the hydrogen flowing through the first flow path; and a water vapor flow path configured to guide the water vapor generated by the ignition portion from the ignition portion to the airbag, wherein before inflating and deploying the airbag, the first opening and closing portion is in the closed state, and at the time of inflating and deploying the airbag, the first opening and closing portion is temporarily switched from the closed state to the opened state to supply the appropriate volume of hydrogen to the ignition portion. an airbag supplied with water vapor to be inflated and deployed; an ignition portion configured to ignite a mixed fluid of hydrogen and oxygen to generate water vapor to be supplied to the airbag; and a first flow path configured to guide hydrogen to the ignition portion from a first container filled with a volume of hydrogen in a pressurized state, the volume of the hydrogen in the first container exceeding an appropriate volume for inflation and deployment of the airbag, characterized by further comprising: a first opening and closing portion disposed in the first flow path and switchable between a closed state where a flow of hydrogen from the first container is shut off and an opened state where the flow of hydrogen from the first container is opened; an introduction section (6, 6A) disposed at a position downstream of the first opening and closing portion in the first flow path, the introduction section (6, 6A) introducing oxygen supplied from an oxygen supply source into the first flow path and thereby mixes the oxygen with the hydrogen flowing through the first flow path; and a water vapor flow path configured to guide the water vapor generated by the ignition portion from the ignition portion to the airbag, wherein before inflating and deploying the airbag, the first opening and closing portion is in the closed state, and at the time of inflating and deploying the airbag, the first opening and closing portion is temporarily switched from the closed state to the opened state to supply the appropriate volume of hydrogen to the ignition portion. a first opening and closing portion disposed in the first flow path and switchable between a closed state where a flow of hydrogen from the first container is shut off and an opened state where the flow of hydrogen from the first container is opened; an introduction section (6, 6A) disposed at a position downstream of the first opening and closing portion in the first flow path, the introduction section (6, 6A) introducing oxygen supplied from an oxygen supply source into the first flow path and thereby mixes the oxygen with the hydrogen flowing through the first flow path; and a water vapor flow path configured to guide the water vapor generated by the ignition portion from the ignition portion to the airbag, wherein before inflating and deploying the airbag, the first opening and closing portion is in the closed state, and at the time of inflating and deploying the airbag, the first opening and closing portion is temporarily switched from the closed state to the opened state to supply the appropriate volume of hydrogen to the ignition portion.