To provide a method for accurately measuring the accumulated height of a deposit in a container even when an object flowing or is being scattered.

An electromagnetic wave transmitter/receiver 7 installed on a furnace generates a microwave signal, transmits it to the inside of the furnace from a microwave antenna 8, and receives a reflection signal from an object (waste matters) in the furnace. The received signal is input to the electromagnetic wave transmitter/receiver 7 to generate and output a detection signal delayed in time according to a distance to the object in signal processing. A signal processor 9 calculates a measurement value representing the detection signal obtained through transmission/reception of the microwave, and further calculates an average of measurement values of the detection signals and outputs it as a measured height of an inner layer of the furnace.