It is expected that hydrogen will serve as a nonpolluting carrier of energy for the next generation of vehicles, and guidelines for its safe use are required. Hydrogen-gas service stations for supplying fuel-cell vehicles will have to handle high-pressure hydrogen gas, but safety regulations for such installations have not received much investigation. In this study, we experimentally investigated the flame characteristics of a rapid leakage of high-pressure hydrogen gas. A hydrogen jet diffusion flame was injected horizontally from convergent nozzles of various diameters between 0.1 and 4 mm at reservoir over pressures of between 0.01 and 40 MPa. The sizes of the flame were measured, and experimental equations were obtained for the length and the width of the flame. Flame sizes depend not only on the nozzle diameter, but also on the spouting pressure. Blow-off limits exists and are determined by the nozzle diameter and the spouting pressure. Furthermore, the radiation from a hydrogen flame can be predicted from the flow rate of the gas and the distance from the flame.