A solution of polymer uniformly dissolved into a solvent is prepared in a solution preparing step. The solvent contains good and poor solvent components for the polymer. The solution is discharged from a coating die to a support to be a coating film thereon in a primary body forming step. A water drop generating step and a fluidity decreasing step are performed in parallel with each other while wet air is applied to the coating film. Water drops are generated on a surface of the coating film in the water drop generating step. The good solvent component is evaporated from the coating film to decrease fluidity of the solution for forming the coating film in the fluidity decreasing step. In the water drop evaporating step, dry air is applied to the coating film. The solvent and the water drops are evaporated from the coating film to obtain the porous film.