An optical measurement instrument for a living body, the instrument comprising: a light source for emitting a plurality of optical signals of different wavelengths to the surface of the living body, wherein the light source includes a plurality of semiconductor light-emitting devices (1 to 3) mounted on one sub-mount for emitting light of a first wavelength between 650 nm and 730 nm, light of a second wavelength of 830 nm or more, and light of a third wavelength in the range 750 nm to 760 nm, a driving circuit connected to the plurality of semiconductor light-emitting devices and adapted to control the optical signal output of the semiconductor light-emitting devices (1 to 3) such as to emit the optical signals of different wavelengths, an optical output power monitoring device adapted to detect the optical signals emitted from the plurality of semiconductor light-emitting devices (1 to 3); a light-receiving device for detecting the plurality of optical signals emitted from the surface of the living body after they have passed through the interior of the living body; and a monitoring signal separation circuit for separating the plurality of optical signals detected by the optical output power monitoring device; wherein the optical signals emitted from the plurality of semiconductor light-emitting devices (1 to 3) are controlled by feeding the optical signals having different wavelengths separated via the monitored signal separation circuit back to the driving circuit connected to the semiconductor light-emitting devices that emit the wavelengths; wherein the semiconductor light-emitting devices and the optical output power monitoring device are housed in one package; and wherein at least one of the light-emitting devices (1 to 3; 130) has an emission layer provided on a GaAs substrate having a lattice constant a and the emission layer includes an In 1-x Ga x As y P 1-y quantum well layer having a lattice constant a w in the surface and a barrier layer, wherein the emission layer is such that the strain ε defined by ε(%)=(a w -a)/a×100 satisfies 0.4%≤ε≤1.4%, wherein y in the composition satisfies 0.10≤y≤0.45, and the wavelength of the emitted light is from 700 nm to 760 nm, or the strain ε defined by ε(%)=(a w -a)/a×100 satisfies 0.4%≤ε≤1.2%, wherein y in the composition satisfies 0.10≤y≤0.25, and the wavelength of the emitted light is from 700 nm to 730 nm, or the strain ε defined by ε(%)=(a w -a)/a×100 satisfies 0.4%≤ε≤0.9%, wherein y in the composition satisfies 0.10≤y≤0.20, and the wavelength of the emitted light is from 700 nm to 720 nm, or the strain ε defined by ε(%)=(a w -a)/a×100 satisfies 0.6%≤ε≤1.4%, wherein y in the composition satisfies 0.20≤y≤0.35, and the wavelength of the emitted light is from 725 nm to 760 nm. a light source for emitting a plurality of optical signals of different wavelengths to the surface of the living body, wherein the light source includes a plurality of semiconductor light-emitting devices (1 to 3) mounted on one sub-mount for emitting light of a first wavelength between 650 nm and 730 nm, light of a second wavelength of 830 nm or more, and light of a third wavelength in the range 750 nm to 760 nm, a driving circuit connected to the plurality of semiconductor light-emitting devices and adapted to control the optical signal output of the semiconductor light-emitting devices (1 to 3) such as to emit the optical signals of different wavelengths, an optical output power monitoring device adapted to detect the optical signals emitted from the plurality of semiconductor light-emitting devices (1 to 3); a light-receiving device for detecting the plurality of optical signals emitted from the surface of the living body after they have passed through the interior of the living body; and a monitoring signal separation circuit for separating the plurality of optical signals detected by the optical output power monitoring device; wherein the optical signals emitted from the plurality of semiconductor light-emitting devices (1 to 3) are controlled by feeding the optical signals having different wavelengths separated via the monitored signal separation circuit back to the driving circuit connected to the semiconductor light-emitting devices that emit the wavelengths; wherein the semiconductor light-emitting devices and the optical output power monitoring device are housed in one package; and wherein at least one of the light-emitting devices (1 to 3; 130) has an emission layer provided on a GaAs substrate having a lattice constant a and the emission layer includes an In 1-x Ga x As y P 1-y quantum well layer having a lattice constant a w in the surface and a barrier layer, wherein the emission layer is such that the strain ε defined by ε(%)=(a w -a)/a×100 satisfies 0.4%≤ε≤1.4%, wherein y in the composition satisfies 0.10≤y≤0.45, and the wavelength of the emitted light is from 700 nm to 760 nm, or the strain ε defined by ε(%)=(a w -a)/a×100 satisfies 0.4%≤ε≤1.2%, wherein y in the composition satisfies 0.10≤y≤0.25, and the wavelength of the emitted light is from 700 nm to 730 nm, or the strain ε defined by ε(%)=(a w -a)/a×100 satisfies 0.4%≤ε≤0.9%, wherein y in the composition satisfies 0.10≤y≤0.20, and the wavelength of the emitted light is from 700 nm to 720 nm, or the strain ε defined by ε(%)=(a w -a)/a×100 satisfies 0.6%≤ε≤1.4%, wherein y in the composition satisfies 0.20≤y≤0.35, and the wavelength of the emitted light is from 725 nm to 760 nm.