A PET apparatus includes a clock unit. The clock unit includes: a time measuring unit that measures a time and a reference time receiving unit that receives a reference time. The PET apparatus also includes a detection time revising unit. By using the reference time received by the reference time receiving unit, the detection time revising unit revises detection times recorded by using the time measured by the time measuring unit. For example, the detection time revising unit revises the detection times by calculating a time error that occurred during an image taking period of a predetermined image taking process by using the time measured by the time measuring unit and the reference time received by the reference time receiving unit and further distributing the calculated time error to each of the detection times recorded during the image taking period.
