To judge a forward direction/a backward direction of a flow quantity without adding another circuit, and to accurately measure the flow quantity in such cases that the flow is zero or in the vicinity of zero.

A phase difference detecting circuit is provided with a pair of comparators, a signal from the one vibration detecting sensor A is compared with a zero level in one comparator A. In the other comparator B, a signal from the other vibration detecting sensor B is compared with a plus prescribed voltage level +VT at the time of rising and compared with a minus prescribed voltage level -VT at the time of falling, and the signal outputs till falling to the minus prescribed voltage level -VT or below when the flow exceeds to the plus prescribed voltage level +VT and outputs till rising to the plus prescribed voltage level +VT or upper when the flow exceeds the minus prescribed voltage level -VT. The phase difference detecting circuit judges a mass flow quantity and a flow direction according to a phase difference that is required by comparing outputs of the two comparators.