An automatic inflatable safety work vest in which a gas is automatically charged into a gas tight chamber forming a floating body upon contacting water, comprises a back part of the vest formed of a non-floating material, a floating body provided at the front of the vest and connected with stretchable bands to the back part and having a gas tight chamber, a pillow integrally formed at the upper portion of the vest which communicates with the gas tight chamber, the pillows being folded over and releasably attached to the outer surface of the back part and a gas charging device in communication with the gas tight chamber and connected to the floating body through a connector, the gas charging device being a flat cylindrical vessel provided with a gas outlet at the bottom surface, a pair of electrodes positioned oppositely and fixed at a side wall near an upper cover portion of the cylindrical vessel, an electric switch provided at the upper portion of the cylindrical vessel and connecting to the electrodes and consisting of a power supply circuit, a charging circuit and a switching circuit, and an inner cylindrical vessel provided at the lower portion of the cylindrical vessel and provided with a bottom plate having a plurality of projections on the outside, a cylindrical perforated side wall having at least one projection and an electric ignition device fixed at the center portion and connecting to the switching circuit, the electric ignition device being in turn surrounded concentrically by, first, a gas generating solid agent and a solid cooling agent.