A light pipe is obtained by laminating on one or both surfaces of a light-transmitting resin plate a polarized-light scattering plate having fine birefringent domains dispersed therein and hence showing anisotropy in scattering. The instrument includes a sensor effective for detecting the analyte, and a barrier film having limited permeability for the analyte covering the inlet orifice on the sensor for reducing the flux of analyte into the sensor. The guide section is disposed on a sensor surface on which the ultrasonic sensor section is provided.