Abstract: An apparatus for measuring water in cow's milk. Sample milk is heated and dryed by microwaves in an oven made of a microwave waveguide. Weights of the sample before and after the drying of the sample are measured by an automatic balance. Weight signals obtained from the balance are memorized in a memory circuit to calculate the solid material ratio or water ratio of the sample milk. A water dummy is employed for stabilizing oven temperature. Support means for one or more additional sample plates may be arranged in the waveguide for preliminary drying of one or more filter paper in the additional sample plate to shorten the subsequent heating time of the sample milk.