NOVELTY - The method involves producing a frame-shaped electrode (3), a separator (4), and a seal preform (6) from uncrosslinked item of solid rubber having adhesiveness. The electrode, separator, and seal preform are arranged in a forming die (7) provided with a pressing element (72). The forming die is closed while pressing side of the electrode along thickness direction using pressing element. The forming die is pressurized and heated to crosslink uncrosslinked item so that seal seals peripheral edge portion of electrode and integrates electrode with the separator.    USE - Manufacturing method of cell assembly for fuel cell (claimed) for power generation and for use as power source for automobile.    ADVANTAGE - Seal preform can be formed easily from uncrosslinked item of the solid rubber so that the cell assembly for fuel cell can be manufactured conveniently within short period of time. Tensile strength and elongation of the solid rubber of seal preform can be maintained so that the necessity of regulating amount of liquid rubber can be avoided and deformation of the electrolyte membrane due to injection pressure of liquid rubber can be avoided. Misalignment of the electrode due to the warping and cracking of the electrode can be suppressed and warping and cracking of the electrode during molding can be avoided. Expansion and contraction of the electrolyte membrane can be controlled by seal so that the durability of the cell assembly can be improved. Peripheral edge portion of electrode can be sealed using seal portion. Positioning of the electrode and seal preform can be performed easily. Side of the electrode can be pressed along thickness direction using pressing element such that the pressing force can be easily obtained by adjusting spring constant of the spring. Sealability of the fuel cell can be ensured so that the operation reliability of the fuel cell can be improved.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for manufacturing method of fuel cell.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the forming die.    Electrode (3)    Separator (4)    Seal preform (6)    Forming die (7)    Pressing element (72)