The waveguiding characteristics in a (Al, Ga) As double‐heterostructure laser, where a built‐in refractive‐index profile in the active layer is introduced by impurity concentration profile, have been investigated under the lasing conditions using the planar stripe laser with deep Zn diffusion. The effective refractive‐index step, which determines the waveguiding mechanism, consists of the built‐in refractive‐index step and a negative index step associated with injected electrons into the active region. It has been found that waveguiding characteristics can be changed from antiguiding tendency to normal guiding by varying the combination of hole and electron concentrations.