Optical topography (OT) based on near infrared spectroscopy (NIRS) is useful for measuring changes in the concentrations of oxygenated hemoglobin (oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb) in brain tissues non-invasively. Conventional OT systems limit subjects' ranges of movement to within the length of optical fibers these systems require for irradiating light on the scalp and detecting light transmitted through the tissues in the human head. We developed a prototype of a multi-channel, portable OT system without optical fibers. We used the portable system to measure changes in the concentrations of oxy-Hb and deoxy-Hb in the frontal cortex while the subjects performed a word fluency task. The results show typical changes in hemoglobin concentrations during the task and suggest the prototype system can be used to investigate brain functions in the frontal cortex.