PROBLEM TO BE SOLVED: To provide a convenient high-performance apparatus for quantitative generation of a VOC gas, capable of quantitatively generating minute amounts of a VOC, easily preventing leakage and wetting of a diffusion filter by VOCs during transportation or the like, and facilitating installation at a measuring site.SOLUTION: The apparatus for quantitative generation of the gas includes: a container storing a volatile organic compound, a cap covering the opening of the container and having a diffusion outlet which causes the gas generated from the volatile organic compound to diffuse out of the container, and a diffusion filter covering the diffusion outlet of the cap and controlling diffusion of the gas. A sintered filter obtained by sintering polymer particles is used as the diffusion filter.