Arthritis, which is associated with local articular rigidity, swelling and pain as well as systemic development of fever and a sense of fatigue, is a disorder leading quickly to deterioration of quality of life. Approximately 300,000 rheumatoid arthritis patients reside in Japan alone1: 1% of the world's population has the disorder worldwide. Although X-ray CT, MRI, and ultrasonic Doppler method are used for examination and diagnosis, various difficulties persist such as radiation exposure, administration of contrast agents, and difficulty in earlier diagnosis and quantitative evaluation. To resolve these difficulties, we developed a handheld photoacoustic imaging system. This study investigated the feasibility of evaluating the degree of inflammation using photoacoustic imaging with multiple wavelengths using in vivo measurements of model rats. Changes in signal intensity depending on the presence or absence of the disorder were examined. Results confirmed that the signal intensity can be intensified at diseased joints. Then, the changes with different time elapsed from drug administration were examined using rats. Results clarified that the degree of inflammation can be evaluated by photoacoustic spectral shapes, which change along with the progress of the inflammation. These analyses verified the usefulness of photoacoustic imaging for diagnosing and evaluating arthritis.