We observed emissions from excited indium (In * ) by optical emission spectroscopy (OES) during InN growth by electron–cyclotron–resonance plasma-excited molecular beam epitaxy (ECR–MBE). The emission intensity of In * was found to increase with the increase in substrate temperature. Considering the decrease in growth rate by increasing substrate temperature, it is suggested that some In which reached the growing surface from the In cell were thermally desorbed and excited when passing through the nitrogen plasma. It was also found that as the positive bias to the substrate increased, the emission intensity of In * decreased. The growth rate of InN with a deeper positive bias was higher than that without bias. These results were successfully explained by considering that effective In supplied to the surface increased by the decrease in In desorption when deeper positive bias was applied. From these observations, it is confirmed that the desorption of In could be characterized by monitoring In * emission intensity. Thus, OES is demonstrated to be useful as a monitoring tool also for InN growth by MBE.