To provide a Coriolis mass flow controller which has improved a measuring method for tube displacement.

This Coriolis mass flow measuring instrument moves a sensor tube by a driving device, detects the phase difference across the tube due to the movement by an optical detecting circuit, and determines a mass flow rate by using linear variation in the phase difference across the tube with the mass flow rate. This constitution reduce the precision load of the phase shift and improves the sensitivity of the mass flow rate measurement. Further, the displacement can be adjusted by using a flat spring, etc., so flow control becomes possible.