The present invention relates to a semiconductor laser diode comprising a GaAs substrate and at least a first conduction-type clad layer, an active layer containing In, Ga and As as component elements, a second conduction-type first clad layer, a current block layer and a second conduction-type second clad layer which are deposited on the substrate in order, wherein a current injection region is formed by said current block layer and said second conduction-type second clad layer, the effective refractive index step &Dgr;neff in the horizontal direction is from 2.5×10−3 to 5.0×10−3 at the emission wavelength, and the width W of the current injection region is from 1.5 to 2.5 &mgr;m. The semiconductor laser diode of the present invention is suited for uses where high output and long life-time are required, such as excitation light source for optical fiber amplifiers.