A manufacturing method of a cell assembly for a fuel cell includes: producing an electrode member, a separator, and a seal member preform, which has a frame shape and which is formed from an uncrosslinked item of solid rubber having adhesiveness, in a predetermined shape in advance; arranging the electrode member, the separator, and the seal member preform in a forming die including a pressing member, and closing the forming die while the pressing member is pressing a side of the electrode member that is opposite to the separator in the thickness direction; and pressurizing and heating the forming die to crosslink the uncrosslinked item so that the seal member seals the peripheral edge portion of the electrode member and integrates the electrode member and the separator with each other.