A highly reliable optical semiconductor device insusceptible to degradation in the characteristics thereof. An n-type buffer layer, n-type first cladding layer, active layer, a p-type first layer of the second cladding layer, p-type etch-stop layer, p-type second layer of the second cladding layer, and p-type contact layer are formed an n-type semiconductor substrate. Two lengths of separation grooves are formed in parallel in such a way as to reach the underside of the p-type second layer of the second cladding layer from the top face of the contact layer, and a ridge is formed between the respective separation grooves. The ridge comprises a lower portion thereof, made up of the second layer of the second cladding layer, and a portion of the contact layer, corresponding to the ridge, made up of the contact layer. Side parts of the top face of the portion of the contact layer, corresponding to the ridge, facing the separation grooves, respectively, are turned to tilted faces, respectively, and a barrier metal layer is formed on top of the tilted faces. Portions extending from side faces of the lower portion of the ridge to run across the respective separation grooves are covered with an insulating film. Since the tilted faces are formed at the respective side parts of the top face of the portion of the contact layer, no stepping occurs to the barrier metal layer. Accordingly, Au of an Au layer formed outside of the barrier metal layer is prevented from being diffused into the portion of the contact layer, corresponding to the ridge, made of GaAs, through steeped parts of the barrier metal layer.