Recent developments in near-infrared spectroscopy (NIRS) have enabled non-invasive clarification of brain functions in psychiatric disorders. Functional neuroimaging studies of patients with obsessive–compulsive disorder (OCD) have suggested that the frontal cortex and subcortical structures may play a role in the pathophysiology of the disorder. Twelve treatment-naïve children with OCD and 12 age- and sex-matched healthy control subjects participated in the present study after giving consent. The relative concentrations of oxyhemoglobin (oxy-Hb) were measured with prefrontal probes every 0.1 s during the Stroop color-word task, using 24-channel NIRS machines. During the Stroop color-word task, the oxy-Hb changes in the OCD group were significantly smaller than those in the control group in the prefrontal cortex, especially in the frontopolar cortex. The present study suggests that children with OCD have reduced prefrontal hemodynamic response as measured by NIRS.