Laser ray transmitting colored thermoplastic resin composition containing a dis-azo salt forming dye consisting of an anionic component from a dis-azo acid dye and an organic ammonium component, said acid dye being represented by Formula (1) below, R.sup.1--N.dbd.N-E-N.dbd.N--R.sup.2; (1) E: (2), (3), (4); R.sup.1, R.sup.2: aryl, pyrazolone; one or more of E, R.sup.1, R.sup.2 have sulfonic acid group, ##STR00001## R.sup.3, R.sup.4: hydrogen atom, alkyl, sulfonic acid group, ##STR00002## R.sup.5, R.sup.6: hydrogen atom, sulfonic acid group, ##STR00003## R.sup.15: sulfonic acid group.Method of laser welding wherein a contact portion of a laser ray transmitting material comprising the laser ray transmitting colored thermoplastic resin composition above and a laser ray absorbing material is welded by irradiating laser ray so that the laser ray passes through the laser ray transmitting material and is absorbed in the laser ray absorbing material with the laser ray transmitting material and the laser ray absorbing material in contact with each other.