The present invention provides an ultra-mini and low cost refractive index measuring device applicable to biochemical measurements of an extremely minute amount of a sample. The refractive index measuring device uses a photonic crystal without any requirement of an external spectrograph or the like.The micro sensor device according to the present invention includes a light source emitting light with a single wavelength, a microcavity in which a resonant wavelength varies depending on a position thereof. A refractive index of a material to be measured is measured based on positional information by detecting a transmitting position of light changing in response to a change of a refractive index of the measured material. The micro sensor device according to the present invention enables measurement of a refractive index of a material to be measured without using a large-scale spectrograph.