Diagnostic method for film thickness sensor, and film thickness monitor

To provide a diagnostic method for a film thickness sensor, said diagnostic method making it possible to correctly determine a service life of a crystal oscillator, and to provide a film thickness monitor. According to one embodiment of the present invention, a diagnostic method for a film thickness sensor is a diagnostic method for a film thickness sensor (14) having a crystal oscillator (20). In the method, the crystal oscillator (20) mounted on a sensor head is oscillated, fluctuation range of change rates of oscillation frequencies of the crystal oscillator (20) is measured, and when the fluctuation range exceeds a predetermined value, it is determined that the crystal oscillator (20) cannot be used. 本发明提供种膜厚传感器的诊断方法以及膜厚监视器，以能够对石英晶体谐振器进行准确的寿命判定。本发明的个方式所涉及的膜厚传感器的诊断方法是具有石英晶体谐振器(20)的膜厚传感器(14)的诊断方法,其中，使安装在传感器头上的石英晶体谐振器(20)振荡,测定石英晶体谐振器(20)的振荡频率的变化率的变动幅度,当上述变动幅度超过规定值时，判定为石英晶体谐振器(20)无法使用。