The SAGA Light Source (SAGA-LS) accelerator consists of 260-MeV injector linac and 1.4-GeV storage ring, and it started a user-mode operation in 2006. In order to measure the size of the stored electron beam, we adopted a synchrotron radiation interferometer, which was invented at KEK-PF about fifteen years ago. In this method, the beam size can be obtained from the contrast (visibility) of interferogram of the synchrotron (visible) light passed through a double slit. Our measurement system consists of a double slit, achromatic lens, ND filter, magnification lens, polarization filter, band-pass filter, and CCD camera. The image of the interferogram was acquired through a frame grabber board in a Windows PC (server). The software was developed on LabVIEW. The interferogram was fitted to a theoretical equation and then the visibility was extracted. The beam size obtained is displayed on a console PC (client) in the control room every 1 s. The client-server system employs the ActiveX CA protocol. This measurement system is useful for the diagnosis of the beam status and the control of the betatron coupling.