Multi-channel NIRS, so-called optical topography (OT), allows functional mapping of the cortex; however, it takes a long time to set optodes on the head and is relatively expensive. Thus, OT is not suitable as a screening test of brain disorders evaluating many subjects. Recently, a wearable two-channel continuous wave NIRS (CW-NIRS) device has been developed. Such a simple NIRS device may be applicable as a screening test of brain disorders; however, its reliability in measurements of brain function is not yet clear. Here, we tested a two-channel CW-NIRS, which employs single LED (800 nm) for measurement of total hemoglobin (t-Hb) changes. We measured t-Hb changes in the bilateral prefrontal cortex (PFC) during mental arithmetic tasks, employing the CW-NIRS and time-resolve NIRS (TNIRS). The left-right asymmetry of the PFC activity was evaluated by calculating the laterality index (LI; (R-L)/(R + L) of t-Hb), which reflects mental stress. The interval between CW-NIRS and TNIRS measurements was 1-13 days. A significant positive correlation was observed between LI measured by CW-NIRS and TNIRS. These results suggest the reliability of the simple CW-NIRS, and it may be applicable to prevent stress-induced various diseases. Finally, it should be emphasized that the left-right asymmetry of PFC activity is relatively stable.