A group III nitride semiconductor laser is provided that has a good optical confinement property and includes an InGaN well layer having good crystal quality.An active layer 19 is provided between a first optical guiding layer 21 and a second optical guiding layer 23. The active layer 19 can include well layers 27a, 27b, and 27c and further includes at least one first barrier layer 29a provided between the well layers. The first and second optical guiding layers 21 and 23 respectively include first and second InGaN regions 21a and 23a smaller than the band gap E29 of the first barrier layer 29a, and hence the average refractive index nGUIDE of the first and second optical guiding layers 21 and 23 can be made larger than the refractive index n29 of the first barrier layer 29a. Thus, good optical confinement is achieved. The band gap E29 of the first barrier layer 29a is larger than the band gaps E21 and E23 of the first and second InGaN regions 21a and 23a.