Contamination is always troublesome in spacecraft development, but it has yet to be investigated because outgassing, transportation, deposition, and re-emission largely depend on spacecraft design. Designing spacecraft requires an accurate estimate of contaminant emission and deposition. Particularly for today’s sophisticated spacecraft, analytical tools would be indispensable—but current simulations need to be improved. Our study addresses the first phase in the occurrence of contamination: improving the outgassing model. We focused on diffusion-limited phenomena because the diffusion rate of outgassing molecules in materials is thought to be much slower than their desorption rate at the surface. A new outgassing test method and analytical procedure was originally devised for extracting the parameters of the diffusion model. In comparing the desorption and diffusion models, we developed a model for the emission of outgassing molecules based on the diffusion theory and updated some functions of Japan’s Spacecraft Induced Contamination Environment analysis software (J-SPICE). From this, we developed J-SPICE2 and installed the diffusion model to the software for calculating outgassing behavior.