Research was conducted on hydrogen diffusion behavior to construct a simulation method forhydrogen leaks into complexly shaped spaces such as around the hydrogen tank of a fuel cell electricvehicle (FCEV). To accurately calculate the hydrogen concentration distribution in the vehicleunderfloor space, it is necessary to take into account the effects of hydrogen mixing and diffusion dueto turbulence. The turbulence phenomena that occur in the event that hydrogen leaks into the vehicleunderfloor space were classified into the three elements of jet flow, wake flow, and wall turbulence.Experiments were conducted for each turbulence element to visualize the flows, and the hydrogenconcentration distributions were measured. These experimental values were then compared withcalculated values to determine the calculation method for each turbulence phenomenon. Accuratecalculations could be performed by using the k-? Shear Stress Transport (SST) model for theturbulence model in the jet flow calculations, and the Reynolds Stress Model (RSM) in the wallturbulence calculations. In addition, it was found that the large fluctuations produced by wake flowcan be expressed by unsteady state calculations with the steady state calculation solutions as the initialvalues. Based on the above information, simulations of hydrogen spouting were conducted for thespace around the hydrogen tank of an FCEV. The hydrogen concentration calculation results matchedclosely with the experimental values, which verified that accurate calculations can be performed evenfor the complex shapes of an FCEV.