A flowmeter includes a flow sensor capable of measuring instantaneous flow rates of forward and backward flows of a fluid, smoothing means for smoothing the measured instantaneous flow rate, an integrating counter for integrating the smoothed flow rate and outputting an integrated flow rate, determination means for monitoring the smoothed flow rate to determine whether or not the fluid is fluctuating, an auxiliary counter for integrating, in place of the integrating counter, the smoothed flow rate while the fluid is judged to be fluctuating by the determination means and when fluctuation of the fluid is detected, and control means for adding an integrated flow rate held by the auxiliary counter to that held by the integrating counter when it is judged by the determination means that the fluid has begun to flow regularly, and causing the integrating counter to restart integrating the smoothed flow rate.