A group III nitride semiconductor optical device includes: a substrate comprising a group III nitride semiconductor; a first group-III nitride semiconductor region on a primary surface of the substrate; a second group-III nitride semiconductor region on the primary surface of the substrate; and an active layer between the first group-III nitride semiconductor region and the second group-III nitride semiconductor region. The primary surface of the substrate tilts at a tilt angle in the range of 63 degrees to smaller than 80 degrees toward the m-axis of the group III nitride semiconductor from a plane perpendicular to a reference axis extending along the c-axis of the group III nitride semiconductor. The first group-III nitride semiconductor region, the active layer, and the second group-III nitride semiconductor region are arranged in the direction of the normal axis to the primary surface of the substrate. The active layer is configured to produce light having a wavelength in the range of 580 nm to 800 nm. The active layer includes an epitaxial semiconductor layer comprising a gallium nitride based semiconductor containing indium as a group III element. The epitaxial semiconductor layer has an indium content ranging from 0.35 to 0.65. The c-axis of the gallium nitride based semiconductor tilts from the normal axis. The reference axis is oriented in the direction of either the axis [0001] or [000−1] of the group III nitride semiconductor.