A platelet aggregation measuring method comprising: preparing a measurement sample which contains a sample and a reagent which includes a platelet activator; mixing the measurement sample at a first speed; mixing the measurement sample at a second speed which is greater than the first speed after mixing the sample at the first speed; obtaining optical information from the measurement sample while mixing the measurement sample at the second speed; and analyzing aggregation of platelets in the sample based on the optical information is disclosed. A platelet aggregation measuring apparatus is also disclosed.