A manufacturing method of a cell assembly for a fuel cell, the cell assembly including: an electrode member having a membrane electrode assembly formed from an electrolyte membrane and a pair of electrode catalyst layers disposed on both sides of the electrolyte membrane in a thickness direction, and a pair of porous layers disposed on both sides of the membrane electrode assembly in the thickness direction; a separator having an electrode member arrangement portion arranged to be stacked with the electrode member and with which one side of the electrode member in the thickness direction is in contact, and a seal member arrangement portion having a frame shape and disposed on an outer side of the electrode member arrangement portion in a planar direction; and a seal member formed from a crosslinked item of solid rubber and disposed in the seal member arrangement portion to seal a peripheral edge portion of the electrode member along a shape of the peripheral edge portion and integrate the electrode member and the separator with each other, the method comprising the step of producing the electrode member, the 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, as a preliminary production step; characterized by the steps of: - arranging the electrode member, the separator, and the seal member preform in a forming die including a pressing member such that the electrode member is housed in a frame of the seal member preform and such that one side of the electrode member in the thickness direction is in contact with the electrode member arrangement portion, and - closing the forming die while the pressing member is pressing the other side of the electrode member in the thickness direction, as a member arrangement step; and - pressurising 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, as an integration step. an electrode member having a membrane electrode assembly formed from an electrolyte membrane and a pair of electrode catalyst layers disposed on both sides of the electrolyte membrane in a thickness direction, and a pair of porous layers disposed on both sides of the membrane electrode assembly in the thickness direction; a separator having an electrode member arrangement portion arranged to be stacked with the electrode member and with which one side of the electrode member in the thickness direction is in contact, and a seal member arrangement portion having a frame shape and disposed on an outer side of the electrode member arrangement portion in a planar direction; and a seal member formed from a crosslinked item of solid rubber and disposed in the seal member arrangement portion to seal a peripheral edge portion of the electrode member along a shape of the peripheral edge portion and integrate the electrode member and the separator with each other, the method comprising the step of producing the electrode member, the 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, as a preliminary production step; characterized by the steps of: - arranging the electrode member, the separator, and the seal member preform in a forming die including a pressing member such that the electrode member is housed in a frame of the seal member preform and such that one side of the electrode member in the thickness direction is in contact with the electrode member arrangement portion, and - closing the forming die while the pressing member is pressing the other side of the electrode member in the thickness direction, as a member arrangement step; and - pressurising 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, as an integration step. - arranging the electrode member, the separator, and the seal member preform in a forming die including a pressing member such that the electrode member is housed in a frame of the seal member preform and such that one side of the electrode member in the thickness direction is in contact with the electrode member arrangement portion, and - closing the forming die while the pressing member is pressing the other side of the electrode member in the thickness direction, as a member arrangement step; and - pressurising 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, as an integration step.