The relation between the superconductivity and the scattering time τ of the quasiparticle in the organic superconductor κ-(BEDT-TTF)2Cu(NCS) 2 has been investigated by measuring the de Haas-van Alphen (dHvA) oscillations. The single crystals substituted partly with BMDT-TTF molecules, κ-[(BEDT-TTF)1-x(BMDT-TTF)x]2Cu(NCS) 2 are synthesized for introducing the nonmagnetic impurities. The oscillation frequency F 600 5 T and the effective mass ratio m* /mo 3.2 0.2 are the same with those of no substituted crystal. On the other hand, the BMDT-TTF molecule substitution reduces the quasiparticle scattering time τ ∼ 2.5 psec for x 0 to ∼ 0.5 psec for x 0.15. The relation between T c and τ in the nonmagnetic molecule substituted crystals is discussed in comparison with the Abrikosov-Gorkov relation.