This invention provides a new therapeutic or prophylactic agent for arthritis such as osteoarthritis. Specifically, it provides a therapeutic or prophylactic agent for arthritis such as osteoarthritis, or an agent for promoting the growth of articular chondrocyte, comprising a guanyl cyclase B (GC-B) activator as an active ingredient; or a method for inhibiting arthritis or for promoting the growth of articular chondrocyte by activating GC-B; or a method for screening an agent for promoting the growth of articular chondrocyte or an agent capable of treating arthritis using the GC-B activity as an indication.