An FBG system with lower power supplied to a temperature controller, while allowing for precise temperature control of an FBG grating. The FBG system includes a high temperature FBG-mounting structure and a low temperature FBG-mounting structure, and a housing containing them. The high temperature FBG-mounting structure includes an FBG module and a thermo module. The temperature of the FBG in the FBG module may be made higher than the environmental temperature by supplying heat from a heat-conducting portion to the thermo module. The low temperature FBG-mounting structure includes an FBG module and a thermo module. The temperature of the FBG in the latter FBG module may be made lower than the environmental temperature by supplying heat from the latter thermo module to the heat-conducting portion. The FBG-mounting structures are provided in parallel on the inner bottom surface of the heat-conducting portion, part of the housing.