Research was conducted on a method for the simulation of the diffusion of pressurized hydrogen leaking at high speed from a small opening into the complexly shaped space of a fuel cell electric vehicle with a practicable calculation time.The fact that the scale of the calculations was large and the calculation time was therefore extended represented issues in relation to this simulation method. The reduction of calculation time through the use of a three part partitioning method was proposed in order to resolve this issue. In this method, the calculation region is divided into three: In the first part, steady-state compressible flow calculations are conducted for the region close to the hydrogen outlet where the Mach number is higher than 0.5. In the second part, steady-state incompressible flow calculations are conducted for the process in which the hydrogen flow spreads and its flow rate declines in the space surrounding the first part space, using the results from the first part as boundary conditions. In the third part, unsteady-state incompressible flow calculations are conducted for all remaining calculation regions, using the results from the second part as boundary conditions.The three part partitioning method was used to simulate the diffusion of hydrogen around the hydrogen tank of a fuel cell electric vehicle. Using the method, it was possible to calculate the behavior of the hydrogen over a 30-second period in four days. In addition, a good quantitative match between calculation values and test values for hydrogen concentration was achieved.