A signal processing method for a Coriolis flowmeter, the signal processing method including: measuring a frequency of a first digital signal obtained by converting a first input signal from one of a pair of vibration detection sensors into the first digital signal; transmitting a modulatable frequency signal based on the measured frequency of the first digital signal; performing frequency conversion to add or subtract the frequency of the modulatable frequency signal to or from the frequency of the first digital signal; performing frequency conversion to add or subtract the frequency of the modulatable frequency signal to or from the frequency of a second digital signal obtained by converting a second input signal from the other one of the pair of vibration detection sensors into the second digital signal; and measuring a phase difference between (i) the frequency converted first digital signal and (ii) the frequency converted second digital signal.