A refractive index sensor senses the refractive index of a mixed liquid and generates a corresponding signal. A temperature sensor senses the temperature of the liquid and generates a corresponding signal. A signal mixer mixes the refractive index signal and the temperature signal with each other to provide a single mixed signal which is then fed through a single signal line to a liquid content calculator where the sensed refractive index is modified based on the sensed liquid temperature to provide a temperature compensated refractive index from which the content of a liquid component in the liquid is calculated. This simplifies the arrangement of the entire apparatus and improves the operational reliability thereof as compared with the case in which the two signals are separately supplied to the liquid content calculator through two signal supply systems.