To determine the basic word order in Japanese, oxyhemoglobin concentration changes in the frontal region of 32 healthy men were measured by near-infrared spectroscopy. Our experiment used four types of sentences: (i) subject-object-verb (SOV), (ii) object-subject-verb (OSV), (iii) subject-naze (why)-object-verb (SwhyOV), and (iv) naze (why)-subject-object-verb (whySOV). The results have shown that although oxyhemoglobin changes in the OSV sentences were significantly larger than those in the SOV sentences in the left frontal lobe, such effects were not observed between SwhyOV sentences and whySOV sentences. These results are consistent with the view that both SwhyOV and whySOV are the basic, which suggests that neurolinguistic evidence has the potential to provide an important basis for determining the basic word order in a free word-order language.