Band gap widening due to impurity contamination such as oxygen (O) incorporation into InN layers has been observed by growth without PBN sealing in the RF-plasma reactor. Furthermore, V/III ratio dependence of the absorption edge of InN layers grown under O-contarninated background has shown a strong correlation with that of the residual carrier concentration. A clearly red-shift of the absorption edge has been also observed for InN layers grown under the simultaneous irradiation of Ga beam. These results strongly indicate that O may be one of the strong candidates of impurity identified as an origin of band gap widening of InN. © 2005 Elsevier B.V. All rights reserved.