A non-destructive detection method, comprising: signal transmission step of transmitting a microwave band constant high-frequency signal to a transmission line and generating outside of the transmission line an electromagnetic field at a position where a test object is at least present; reference measurement step (S2, S3) of measuring the microwave band constant high-frequency signal transmitted through the transmission line and setting a value which is measured firstly as a reference value; measurement step (S4, S5) of measuring the microwave band constant high-frequency signal transmitted through the transmission line while moving the position of the test object and obtaining a value which is measured secondly or later; gradient calculation step of calculating a gradient of a straight line at a predetermined 2D position coordinate based on a difference between the reference value and the value which is measured secondly or later, wherein the predetermined 2D position coordinate is a 2D position coordinate whose vertical axis is one of real and imaginary parts of a S parameter of the microwave band constant high-frequency signal expressed by complex notation or one of phase and amplitude obtained by calculating the real and imaginary parts and whose horizontal axis is the other of the real and imaginary parts or the other of the phase and amplitude, wherein the S parameter is a transmission coefficient indicating voltage ratio of output signal to input signal; and judgment step comprising the steps of: comparing a reference gradient with the gradient which is calculated in the gradient calculation step, the reference gradient is the gradient of the straight line at the predetermined 2D position coordinate which was calculated in advance based on the measured value of the microwave band constant high-frequency signal from the transmission line when the microwave band constant high-frequency signal is transmitted to the transmission line and also the electromagnetic field of the transmission line is generated at a position where a known test object exists, the known test object was confirmed in advance that no foreign substance is mixed; and when the reference gradient is different from the calculated gradient, judging that a foreign substance is mixed in the test object. signal transmission step of transmitting a microwave band constant high-frequency signal to a transmission line and generating outside of the transmission line an electromagnetic field at a position where a test object is at least present; reference measurement step (S2, S3) of measuring the microwave band constant high-frequency signal transmitted through the transmission line and setting a value which is measured firstly as a reference value; measurement step (S4, S5) of measuring the microwave band constant high-frequency signal transmitted through the transmission line while moving the position of the test object and obtaining a value which is measured secondly or later; gradient calculation step of calculating a gradient of a straight line at a predetermined 2D position coordinate based on a difference between the reference value and the value which is measured secondly or later, wherein the predetermined 2D position coordinate is a 2D position coordinate whose vertical axis is one of real and imaginary parts of a S parameter of the microwave band constant high-frequency signal expressed by complex notation or one of phase and amplitude obtained by calculating the real and imaginary parts and whose horizontal axis is the other of the real and imaginary parts or the other of the phase and amplitude, wherein the S parameter is a transmission coefficient indicating voltage ratio of output signal to input signal; and judgment step comprising the steps of: comparing a reference gradient with the gradient which is calculated in the gradient calculation step, the reference gradient is the gradient of the straight line at the predetermined 2D position coordinate which was calculated in advance based on the measured value of the microwave band constant high-frequency signal from the transmission line when the microwave band constant high-frequency signal is transmitted to the transmission line and also the electromagnetic field of the transmission line is generated at a position where a known test object exists, the known test object was confirmed in advance that no foreign substance is mixed; and when the reference gradient is different from the calculated gradient, judging that a foreign substance is mixed in the test object. comparing a reference gradient with the gradient which is calculated in the gradient calculation step, the reference gradient is the gradient of the straight line at the predetermined 2D position coordinate which was calculated in advance based on the measured value of the microwave band constant high-frequency signal from the transmission line when the microwave band constant high-frequency signal is transmitted to the transmission line and also the electromagnetic field of the transmission line is generated at a position where a known test object exists, the known test object was confirmed in advance that no foreign substance is mixed; and when the reference gradient is different from the calculated gradient, judging that a foreign substance is mixed in the test object.