The semiconductor device fabrication method comprises the step of forming a first porous insulation film 38 over a semiconductor substrate 10; the step of forming a second insulation film 40 whose density is higher than that of the first porous insulation film 38; and the step of applying electron beams, UV rays or plasmas with the second insulation film 40 present to the first porous insulation film 38 to cure the first porous insulation film 38. The electron rays, etc. are applied to the first porous insulation film 38 through the denser second insulation film 40, whereby the first porous insulation film 38 can be cured without being damaged. The first porous insulation film 38 can be kept from being damaged, whereby the moisture absorbency and density increase can be prevented, and resultantly the dielectric constant increase can be prevented. Thus, the present invention can provide a semiconductor device including an insulation film of low dielectric constant and high mechanical strength.