A radio signal transmitter receives, from an image display device (i) synchronizing signals for switching between displaying a right-eye image and displaying a left-eye image on the image display device and (ii) liquid crystal shutter control data indicating which image is displayed, and transmits the synchronizing signals and the liquid crystal shutter control data to stereoscopic image glasses. The radio signal transmitter includes: a period detecting section that detects a period of each of the synchronizing signals; and a period data storage section in which the period is stored as period data. The radio signal transmitter is configured such that the period data in the period data storage section is updated every time the period of each of the synchronizing signals is detected. This makes it possible to provide a radio signal transmitter, stereoscopic image glasses, and an image display system that achieve a reduction in power consumption.