A master disk which simulates a tire and is used at the time of confirmation or correction of measurement accuracy of a geometry measuring device for measuring the surface shape of a tire, of a tire testing machine, includes: a simulated tire section simulating a tire; a lower tubular section having a cylindrical shape, in which the tip side thereof is connected to the lower end side of the simulated tire section so as to be coaxial with the simulated tire section and a guide surface inclined in a tapered shape so as to have a larger diameter toward the base end side is formed on the base end side of an inner peripheral surface; and a neck section and a flange section which are provided on the upper end side of the simulated tire section and detachably engaged with test rim moving and exchanging means of a tire holding device of the tire testing machine.