We numerically investigated the initial behavior of leakage and diffusion from high-pressure hydrogen storage tank assumed in hydrogen station. First, calculations are carried out to validate the present numerical approach and compare with the theoretical distribution of hydrogen mass fraction to the direction, which is vertical to the jet direction, in the case of hydrogen leaking out from the circular injection port, whose diameter is 0.25 mm. Then, performing calculations about hydrogen leakage and diffusion behavior on different tank pressures, the effects are examined to reduce damage by gas explosion assumed in the hydrogen station. There is no significant difference in the diffusion distance to the jet direction from a start to 0.2 ms. After 0.2 ms, it is seen the difference in the diffusion distance to the jet direction in different pressure. As tank pressures become large, the hydrogen diffusion not only to the jet direction but also to the direction, which is vertical to the jet direction, is remarkably seen. Then, according to histories of the percentage of the flammable mass to total one in the space, it drastically increases up to 30%2between 0 and 0.05 ms. After 0.05 ms, it uniformly increases, so it is shown that the explosion risk becomes high over time. The place where mass within flammability range distributes at a certain time is shown. Hydrogen widely diffuses to jet direction and distributes in each case and time. Therefore, it is found that when it is assumed that ignition occurs by some sources in place where high-pressure hydrogen is leaked and diffused, the magnitude of the explosion damage can be predicted when and where ignition occurs.