PROBLEM TO BE SOLVED: To accurately measure the concentration or flow rate of powder or fluid by detecting only direct waves in transmission microwaves.SOLUTION: A method for measuring the concentration or the flow rate comprises the steps of: while transmitting/receiving microwaves along a diameter inside of a pipe, sweeping a transmission frequency of a transmitted microwave and mixing a part of the transmitted microwave with a received microwave to generate a beat signal; from a complex spectrum obtained by performing Fourier transform on the beat signal, calculating a distance spectrum indicating a relation between the magnitude of the complex spectrum and a propagation distance of microwaves, and a phase spectrum indicating a relation between the initial phase thereof and the propagation distance of microwaves, selecting a specific peak appearing at a position corresponding to the inner diameter of the pipe in the distance spectrum, and determining a specific initial phase corresponding to the specific p
