NOVELTY - A laser transmitting thermoplastic resin member (13) and a porous member (12) are joined by laminating the laser transmitting thermoplastic resin member and the porous member; emitting a laser beam at a side of the resin member; heating the porous member to melt the resin member; impregnating the melted resin into voids of the porous member; and cooling to solidify the resin.    USE - For joining a laser transmitting thermoplastic resin member and a porous member used in fuel cell (claimed).    ADVANTAGE - The inventive method can optimize laser transmission efficiency of the thermoplastic resin, a melting point of the thermoplastic resin, energy density of the laser, and an emitting speed of the laser.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a thermoplastic resin joining method for melting and joining an interface between laser transmitting thermoplastic resins composed of resin or resin and fillers by means of laser emission; and    (2) a fuel cell in which electric cells constituted so that a membrane electrode assembly (MEA) having a catalytic layer and a diffusion layer on both surfaces of an electrolytic membrane is held between a pair of separators are laminated, where the separators have a metallic internal member and thermoplastic resin provided to an outer peripheral portion of the metallic internal member, and the thermoplastic resins are joined by the above specified joining method so that the separators are joined.    Energy of the laser is designated by W(w), a laser emitting speed is designated by S (mm/s), a laser spot diameter is designated by D (mm), laser transmission efficiency of the thermoplastic resin is designated by Tr, a melting thickness of the thermoplastic resin is designated by Th (mm), specific gravity of the thermoplastic resin is designated by SG (g/cm3), a melting point of the thermoplastic resin is designated by MP (K), and specific heat at constant pressure of the thermoplastic resin is designated by Cp (kJ/kg-K), so that the equation 0.7less than or equal toWxTr/SxDxThxSGx10-3xMP\Cpless than or equal to1.3, is satisfied.    DESCRIPTION OF DRAWING(S) - The figure is a sectional view illustrating the joined body obtained by the joining method of the invention.    Thermoplastic resin member (11)    Porous member (12)    Laser transmitting thermoplastic resin member (13)    Laser transmitter (14)    Condensing lens (15)