A microwave heating apparatus comprising: a heating chamber configured to accommodate an object to be heated; a microwave generator configured to generate a microwave to be supplied to the heating chamber; a waveguide (4, 40) through which the microwave generated by the microwave generator propagates to the heating chamber; a reflected wave detector configured to detect at least part of a reflected wave inside the waveguide (4, 40); a quantity determiner configured to determine the quantity of the object, from a reflected wave detection amount that is an amount of the at least part of the reflected wave detected by the reflected wave detector; and a controller configured to control the microwave generator, based on the quantity determined by the quantity determiner, characterized in that: the quantity determiner is further configured to determine the quantity of the object by comparing the reflected wave detection amount detected by the reflected wave detector during heating with the reflected wave detection amount detected by the reflected wave detector when the object is absent, and the microwave heating apparatus further comprises: a radiation antenna configured to radiate the microwave propagating inside the waveguide (4, 40) to the heating chamber; and a rotary driver configured to rotate the radiation antenna, wherein the controller is further configured to control the direction in which the radiation antenna faces by controlling driving of the rotary driver, and the quantity determiner is further configured to determine the quantity of the object when the radiation antenna faces in the direction in which the difference between the compared reflected wave detection amounts is maximized. a heating chamber configured to accommodate an object to be heated; a microwave generator configured to generate a microwave to be supplied to the heating chamber; a waveguide (4, 40) through which the microwave generated by the microwave generator propagates to the heating chamber; a reflected wave detector configured to detect at least part of a reflected wave inside the waveguide (4, 40); a quantity determiner configured to determine the quantity of the object, from a reflected wave detection amount that is an amount of the at least part of the reflected wave detected by the reflected wave detector; and a controller configured to control the microwave generator, based on the quantity determined by the quantity determiner, characterized in that: the quantity determiner is further configured to determine the quantity of the object by comparing the reflected wave detection amount detected by the reflected wave detector during heating with the reflected wave detection amount detected by the reflected wave detector when the object is absent, and the microwave heating apparatus further comprises: a radiation antenna configured to radiate the microwave propagating inside the waveguide (4, 40) to the heating chamber; and a rotary driver configured to rotate the radiation antenna, wherein the controller is further configured to control the direction in which the radiation antenna faces by controlling driving of the rotary driver, and the quantity determiner is further configured to determine the quantity of the object when the radiation antenna faces in the direction in which the difference between the compared reflected wave detection amounts is maximized. the quantity determiner is further configured to determine the quantity of the object by comparing the reflected wave detection amount detected by the reflected wave detector during heating with the reflected wave detection amount detected by the reflected wave detector when the object is absent, and the microwave heating apparatus further comprises: a radiation antenna configured to radiate the microwave propagating inside the waveguide (4, 40) to the heating chamber; and a rotary driver configured to rotate the radiation antenna, wherein the controller is further configured to control the direction in which the radiation antenna faces by controlling driving of the rotary driver, and the quantity determiner is further configured to determine the quantity of the object when the radiation antenna faces in the direction in which the difference between the compared reflected wave detection amounts is maximized.