CAPTCHA is an authentication test that distinguishes between humans and machines. CAPTCHA is essential to prevent fraudulent account acquisition by BOTs. Conventional CAPTCHAs were based on the assumption that human abilities were superior to BOTs, but with the evolution of AI technology, this assumption no longer holds true. As CAPTCHAs have become more complex, usability has been compromised. The Munker illusion is a visual illusion in which shapes of the same color appear to be different colors to the human eye due to the synergistic effect of color assimilation and color contrast. In this paper, we propose a new CAPTCHA using the Munker illusion.