To simulate hydrogen outgassing in an ultrahigh vacuum (UHV) chamber and a fusion device, an outgassing theory based on the recombination limited model is presented, where the time evolution of hydrogen concentration in the wall of the vacuum chamber is described by a one-dimensional diffusion equation. The hydrogen outgassing rate is expressed as a boundary condition in the diffusion equation. For the UHV chamber the effect of surface treatment, such as baking and oxidation, and the benefits of a thin-walled chamber for reduction of outgassing rate are examined, and it is shown that the present theory explains well the experimental results which have been reported on these matters. For the fusion device the time evolution of after-shot outgassing observed in the large helical device is simulated. For the simulation, it is essential to know first the initial concentration profile of hydrogen retained in the first wall immediately after the discharge shot. The profile is constructed by referring the calculated depth distribution of hydrogen implanted into Pd using the TRIM code. As a result good agreement between the experimental hydrogen release rate and the calculated one is shown.