Optical tomography (OT) is a method for visualizing brain functions noninvasively. In an OT measurement system, near- infrared light, to which living tissue is highly permeable, is irradiated from the scale of the subject, and the scattered light reflected from the cerebral cortex is detected elsewhere on the scalp. The spatio-temporal blood volume change in the cortex is visualized based on the signal detected using two-dimensionally arranged optodes. The measurement imposes few constraints on the subject, either physically or mentally, thus the subject is in a natural and relaxed condition during measurement. Here we describe our OT system, then report on an experiment to evaluate the system using a phantom. We found that OT can accurately locate the activated region in the cortex. Also, as an example of a clinical application of OT, we used our system to measure the language function, demonstrating the system's ability to measure the activity of Broca's area.