It is an object of the invention to provide an agitation method for sufficiently agitating a mixture of a solution such as a buffer solution and a material to be detected, with no need of any dedicated unit for agitation, for measurements using a quartz crystal oscillator. The method for agitating a liquefied material using a quartz crystal oscillator in vibrating the quartz crystal oscillator at a given frequency and measuring the variation of the frequency due to a substance in contact with the quartz crystal oscillator, is that the quartz crystal oscillator is vibrated at other frequency different from the given frequency and equal to or higher than the fundamental vibration frequency to agitate a liquid containing the substance.