An amplitude detecting method of detecting an amplitude of a waveform of a natural vibration of a tester main body superimposed on test data based on a signal detected by a force detector in a material test executed by a material tester as a high-speed tension test of rapidly applying a shocking tensile test force to a test subject by driving a load mechanism, the tester main body comprising a table, an upper chuck and a lower chuck as a pair of chucks in a configuration for executing a tension test for rapidly separating the pair of chucks which grip both end portions of the test subject, and the lower chuck being connected to the table through the force detector, the amplitude detecting method further comprising: providing a control device that processes a signal detected by the force detector in the material test; wherein the method includes, by a hammer, hitting the lower chuck connected to the force detector and detecting, by a vibration detection device of the material tester, a vibration frequency of the tester main body in a state in which the material test is not executed and storing in a storage device the frequency of the natural vibration of the tester main body; by a period determining part of the control device, acquiring one period of a waveform of the natural vibration of the tester main body from a reciprocal of the stored frequency of the natural vibration of the tester main body; by a maximum value/minimum value calculating part of the control device, acquiring a maximum value and a minimum value of the amplitude of the waveform of the natural vibration of the tester main body superimposed on the test data in data of a time period in which there is a state in which the test force is applied to the test subject in the test data; and by an amplitude determining part of the control device, determining the amplitude of the natural vibration of the tester main body on a basis of a difference between the maximum value and the minimum value acquired by the maximum value/minimum value calculating part; wherein the maximum value and the minimum value are acquired in units of a time interval of one period acquired by the period determining part. providing a control device that processes a signal detected by the force detector in the material test; wherein the method includes, by a hammer, hitting the lower chuck connected to the force detector and detecting, by a vibration detection device of the material tester, a vibration frequency of the tester main body in a state in which the material test is not executed and storing in a storage device the frequency of the natural vibration of the tester main body; by a period determining part of the control device, acquiring one period of a waveform of the natural vibration of the tester main body from a reciprocal of the stored frequency of the natural vibration of the tester main body; by a maximum value/minimum value calculating part of the control device, acquiring a maximum value and a minimum value of the amplitude of the waveform of the natural vibration of the tester main body superimposed on the test data in data of a time period in which there is a state in which the test force is applied to the test subject in the test data; and by an amplitude determining part of the control device, determining the amplitude of the natural vibration of the tester main body on a basis of a difference between the maximum value and the minimum value acquired by the maximum value/minimum value calculating part; wherein the maximum value and the minimum value are acquired in units of a time interval of one period acquired by the period determining part.