A wearable optical topography system was designed in order to widen the application fields of non-invasive brain-function measurement. The system incorporates new concepts and technologies to reduce size, weight, and power consumption. The target region for observation is the prefrontal cortex. Twenty-two measurement points were arranged in the headset cover the entire prefrontal region. A wireless LAN allows the simultaneous measurement of multiple subjects. The prototype successfully works as designed. The wearable optical topography system will make it easy to monitor brain activity will be especially useful in the fields of preventive medicine, rehabilitation, cognitive science, and neuromarketing.