Functional near-infrared spectroscopy (fNIRS) is a non-invasive imaging technique which uses near-infrared light (700-900 nm) to monitor functional activities of the brain. With this method, concentration changes of Hb and HbO2 can be detected during cognitive tasks. In this paper, designing of a CW (continuous-wave) 4-channel fNIRS device for monitoring of hemodynamic responses of human brain in middle prefrontal cortex has been discussed. Additionally the same device has been used to study the working memory load in an N-bank task.