A method and apparatus for welding members formed of thermoplastic resin material, with high welding strength and small strength variations. A laser welding apparatus 1 brings a first member 2 formed of a transmissive thermoplastic resin that transmits a laser beam into contact with a second member 3 formed of an absorptive thermoplastic resin that absorbs the laser beam, and joins the thermoplastic resin members by melting their contact surfaces 4 with the laser beam. The laser welding apparatus 1 further comprises a laser beam generator 10 for irradiating the contact surfaces with the laser beam shone from the side of the first member 3 so as to melt at least one of the contact surfaces of the first member and the second member. The apparatus also comprises a clamp mechanism 20 for pressing the first member and the second member together, a temperature sensor 31 as an adjusting means for adjusting the pressing force applied to the first member and the second member, and a personal computer 32 for calculating the pressing force based on the output of the temperature sensor. The clamp mechanism is controlled with the calculated pressing force.