To achieve sustainable human-agent interaction (HAI), we proposed a new model of intrinsically motivated adaptive agent, which learns about the human partner and behaves to satisfy its intrinsic motivation. To investigate the model's effectiveness, we conducted a comparative HAI experiment with a simple interaction setting. The results showed that the model was effective in inducing subjective impressions of higher enjoyability and sustainability. The subjects' brain activity measured by near-infrared spectroscopy (NIRS) indicated higher variability of activity at left dorsolateral prefrontal cortex during the interaction with the proposed agent.