This paper reports the performance of an AC-voltage sensor with a LiNbO3 integrated retroreflective structure based on the Y-junction Mach-Zehnder interferometer. This structure is capable of realizing a low-cost sensor chip because of the small chip size and single optical-fiber connection. In the sensitivity and frequency response evaluation, detection sensitivities of 6.3 µ V / Hz have been measured with a frequency response from 6 Hz to 2 GHz. These measurement limitations were also analyzed theoretically and compared with the experimental results. This unique sensor enables precise voltage measurement in an EMI environment, even inside a computer.