Hydrogen is one of the important alternative fuels for future transportation. At the present stage,research into hydrogen safety and designing risk mitigation measures are significant task. For compactstorage of hydrogen in fuel cell vehicles, storage of hydrogen under high pressure, up to 40 MPa, atrefueling stations is planned, and safety in handling such high-pressure hydrogen is essential. Thispaper describes our experimental investigation into dispersion of high-pressure hydrogen gas whichleaks through pinholes in the piping to the atmosphere. First, in order to comprehend the basic behavior of the steady dispersion of high-pressure hydrogengas from the pinholes, the time-averaged concentrations were measured. In our experiments, initialrelease pressures of hydrogen gas were set at 20 MPa or 40 MPa, and release diameters were in therange from 0.25 mm to 2 mm. The experimental results show that the hydrogen concentration alongthe axis of the dispersion plume can be expressed as a simple formula which is a function of thedownwind distance X and the equivalent release diameter ? . This formula enables us to easilyestimate the axial concentration (maximum concentration) at each downstream distance. However, in order for the safety of flammable gas dispersion to be analyzed, comparisons betweentime-averaged concentrations evaluated as above and lower flammable limit are insufficient. This isbecause even if time-averaged concentration is lower than the flammability limit, instantaneousconcentrations fluctuate, and a higher instantaneous concentration occasionally appears due toturbulence. Therefore, the time-averaged concentration value which can be used as a threshold forassessing safety must be determined considering concentration fluctuations. Once the threshold valueis determined, the safe distance from the leakage point can be evaluated by the above-mentionedsimple formula. To clarify the phenomenon of concentration fluctuations, instantaneous concentrationswere measured with the fast-response flame ionization detector. A small amount of methane gas wasmixed into the hydrogen as a tracer gas for this measurement. The relationship between the time-mean concentration and the occurrence probability of flammable concentration was analyzed. Underthe same conditions, spark-ignition experiments were also conducted, and the relationship between theoccurrence probability of flammable concentration and actual ignition probabilities were alsoinvestigated. The experimental results show that there is a clear correlation between the time-meanconcentration, the occurrence probability of flammable concentration, flame length and occurrenceprobability of hydrogen flame.