© 2019 SPIE-OSA. Associated with local articular rigidity, swelling and pain as well as systemic development of fever and a sense of fatigue, arthritis is a disorder causing deterioration of patients' quality of life. Approximately 300,000 patients report rheumatoid arthritis, one of the typical chronic joint inflammation, in Japan alone: 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 problems exist 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. In this study, the feasibility of evaluating the degree of inflammation using photoacoustic imaging with multiple wavelengths was investigated with in vivo measurements of model rats. The changes in signal intensity depending on the presence or absence of the disorder was examined. It was confirmed that the signal intensity can be intensified at diseased joints. Then, the changes with different time elapsed from drug administration was examined using rats. It was clarified that the degree of inflammation can be evaluated by shapes of photoacoustic spectra, which changed along with the progress of the inflammation. Through these analyses, we verified the usefulness of photoacoustic imaging for the diagnosis and evaluation of arthritis.