A microwave-heating apparatus comprising a high-power microwave oscillator which rises in its output power in a shorter time required than a time for irradiating a testing animal, a waveguide connected to the high-power microwave oscillator, a measuring member for measuring reflection coefficient connected to the waveguide and capable of being separated from the waveguide, an impedance matching part connected to the output part of the waveguide, an applicator formed by a short-circuited tapered rectangular waveguide connected to the output part of the impedance matching part, and a container for fixing a head of the animal which is inserted as a load peeping into the applicator through an insertion port provided in the wall of the applicator, whereby the measuring member for measuring reflection coefficient operates while the high-power microwave oscillator is switched off. According to the apparatus, the heads of animals can be heated under the same heating condition so as to obtain repeatable data.