A method and apparatus for laser welding two members made of thermoplastic resin material whereby high welding strength can be achieved and strength variations can be reduced. A first member 2 comprised of a transmissive thermoplastic resin that transmits a laser beam is brought into contact with a second member 3 comprised of an absorptive thermoplastic resin that absorbs a laser beam. Contact surfaces 4 are melted with a laser beam so as to join the two members. At least the contact surfaces 4 of the two members are pre-heated by a pre-heating means 20 at a temperature lower than the melting temperature of the contact surfaces. The contact surfaces 4 are then irradiated with a laser beam R generated by a laser beam generator 10 that is shone from the side of the first member so as to melt at least one of the contact surfaces of the two members. The melted portion is then post-heated by a post-heating means 20A at a temperature lower than the melting temperature of the melted portion, thereby allowing the melted portion to slowly cool down. The pre-heating and the post-heating are preferably conducted at temperatures above the glass transition temperature of the two members.