A signal processing method for a Coriolis flowmeter which comprises: - at least one flow tube or a pair of flow tubes (2, 3) which is included in a measurement flow tube, - a driving device using a vibrator to drive the at least one flow tube or the pair of flow tubes (2, 3), - velocity sensors or acceleration sensors which are a pair of vibration detection sensors (7, 8) provided on a right side and a left side of the at least one flow tube or one is provided on the right sides and one is provided on the left sides of the pair of flow tubes (2, 3), the pair of vibration detection sensors (7, 8) comprises a first and a second vibration detection sensor (7, 8), - a frequency measurement unit for obtaining a vibration frequency to obtain a density of the fluid, - a phase difference measurement unit for determining a phase difference to obtain a mass flow rate of a fluid, the signal processing method being characterized by: - converting analog signals output from the pair of vibration detection sensors (7, 8) into a first and second digital signal, thereby obtaining a first sin(θ) signal from the first vibration detection sensor and a second sin (θ+δθ) signal from the second vibration detection sensor; - measuring the frequency θ of the digital signal by the frequency measurement unit; - outputting the frequency θ to a transmitter; - determining a frequency θ/N, wherein N is an integer based on the measured frequency θ; - generating a frequency θxn satisfying the following expression θxn = θ (1-1/N); - generating a cos(θxn) signal; - inputting the cos(θxn) signal to a first and a second quadrature modulator (110, 130); - inputting first sin(θ) signal to the first quadrature modulator; - inputting second sin(θ+δθ) signal to the second quadrature modulator; - generating a sin(θ-θxn) signal by the first quadrature modulator, wherein sin(θ-θxn) = sin(θ/N); - generating a sin(θ-θxn+δθ) signal by the second quadrature modulator, wherein sin(θ-θxn + δθ) = sin(θ/N+ δθ), δθ being a phase difference; - inputting the sin(θ-θxn) signal and the sin(θ-θxn+δθ) signal to the phase difference measurement unit; - at least one flow tube or a pair of flow tubes (2, 3) which is included in a measurement flow tube, - a driving device using a vibrator to drive the at least one flow tube or the pair of flow tubes (2, 3), - velocity sensors or acceleration sensors which are a pair of vibration detection sensors (7, 8) provided on a right side and a left side of the at least one flow tube or one is provided on the right sides and one is provided on the left sides of the pair of flow tubes (2, 3), the pair of vibration detection sensors (7, 8) comprises a first and a second vibration detection sensor (7, 8), - a frequency measurement unit for obtaining a vibration frequency to obtain a density of the fluid, - a phase difference measurement unit for determining a phase difference to obtain a mass flow rate of a fluid, - converting analog signals output from the pair of vibration detection sensors (7, 8) into a first and second digital signal, thereby obtaining a first sin(θ) signal from the first vibration detection sensor and a second sin (θ+δθ) signal from the second vibration detection sensor; - measuring the frequency θ of the digital signal by the frequency measurement unit; - outputting the frequency θ to a transmitter; - determining a frequency θ/N, wherein N is an integer based on the measured frequency θ; - generating a frequency θxn satisfying the following expression θxn = θ (1-1/N); - generating a cos(θxn) signal; - inputting the cos(θxn) signal to a first and a second quadrature modulator (110, 130); - inputting first sin(θ) signal to the first quadrature modulator; - inputting second sin(θ+δθ) signal to the second quadrature modulator; - generating a sin(θ-θxn) signal by the first quadrature modulator, wherein sin(θ-θxn) = sin(θ/N); - generating a sin(θ-θxn+δθ) signal by the second quadrature modulator, wherein sin(θ-θxn + δθ) = sin(θ/N+ δθ), δθ being a phase difference; - inputting the sin(θ-θxn) signal and the sin(θ-θxn+δθ) signal to the phase difference measurement unit;