The present invention provides a method for inhibiting the activation of telomerase and an agent for inhibiting the activation of telomerase, a method for inhibiting telomerase activity and an agent for inhibiting telomerase activity, a method for preventing and/or a method for treating a cancer disease, an agent for preventing and/or an agent for treating a cancer disease, all of which comprises inhibiting the binding of MAPKAPK3 to TERT that is a catalytic subunit of telomerase or inhibiting the phosphorylation of TERT by active MAPKAPK3; a method of identifying a compound that inhibits the binding of MAPKAPK3 to TERT or a compound that inhibits the phosphorylation of TERT by active MAPKAPK3; and a reagent kit.