Provided are an agent for noninvasively and easily diagnosing Alzheimer's disease, a method for diagnosing Alzheimer's disease through the measurement, and a measurement kit therefor. The present invention relates to a reagent for measuring soluble GPVI (sGPVI) in a human body fluid; an agent for specifically diagnosing Alzheimer's disease, said agent comprising a reagent for measuring the activation of platelets or coagulation-fibrinolysis system in said human body fluid; and a method for specifically diagnosing Alzheimer's disease, said method comprising a step for measuring the concentration of sGPVI in a collected human body fluid and a step for measuring the activation of platelets or coagulation-fibrinolysis system in said human body fluid.