Disclosed is a method for determining sensitivity to a CDK4/6 inhibitor, comprising the steps of: comparing a value based on activity of at least one CDK selected from CDK4 and CDK6 in a sample collected from a subject, with a threshold level corresponding to the CDK, and determining that the subject is insensitive to the CDK4/6 inhibitor when the value based on the CDK activity is less than the threshold level.