A non-intrusive and quantitative method and apparatus for diagnosing sleep apnea and detecting apnea events by monitoring during sleep abdominal effort and thoracic effort, determining the phase of each effort, determining the difference in phase between each type of effort, and then determining the rate of phase angle change and standard deviation over time. One of the tanks has an inlet tank portion and an outlet tank portion for the introduction and the discharge of a heat exchange medium to the heat exchanger. The diameter of metal powder ranges from 1 \u03bcm to 3 \u03bcm. If the first error signal has been input to a frequency controller a preset number of times or more during an interval before the second error signal is input thereto, the controller instructs a frequency modulator to increase the frequency of the frequency-modulated signal.