A Coriolis mass flowmeter includes a vibrator configured to vibrate a measurement conduct line, an upstream sensor and a downstream sensor, which are placed at a certain distance in the measurement conduct line and are configured to detect the vibration of the measurement conduct line, a first effective value reception section configured to receive an effective value of the first signal, a second effective value reception section configured to receive an effective value of the second signal, a ratio computation section configured to receive the effective value of the first signal and the effective value of the second signal and to compute the ratio of both values, and an anomaly diagnosis section configured to make a comparison between the computed ratio and a certain value and to generate an alarm.