A fiber Bragg grating (FBG), which is formed in a fiber core by introducing a periodic variation of the refractive index reflects selectively a particular wavelength of light due to the Bragg reflection. The reflected optical power is modulated if sound pressure is applied to the FBG. Using the FBG which has such a feature we made the fundamental experiments on the detection of underwater sound. Good linearity is obtained under various conditions with the insertion of optical isolators that avoid the fluctuation of detected output due to the Fabry-Perot interference effect between the FBG and the various facets. The sensitivity of detection is maximized if the slope of the curve in the transmittance-wavelength characteristics is steepest at the operating wavelength. Simultaneous multiplex detection is made possible by wavelength division multiplexing with two FBGs in series connection.