An object of the present invention is to reliably detect a belt to form a base tire having a desired shape and dimensions even in a case where a belt layer or a belt reinforcing layer is made of a non-metal material. Specifically, the present invention provides a method of producing a retread tire by grinding and removing by a grinding means tread rubber of a used tire including a belt layer, the tread rubber being disposed on the outer side in the radial direction of the belt layer, comprising the steps of: measuring hardness of the tread rubber by a hardness measuring means; and completing grinding of the tread rubber by the grinding means when hardness of the tread rubber measured by the hardness measuring means has reached a predetermined hardness.