To provide a processing method for reducing the thickness of a workpiece, while also flattening it with high accuracy.

A processing method for flattening, by grinding, a workpiece bonded on a substrate 23 comprises: a substrate holding step for holding the substrate by a chuck table 50 of a grinder; a substrate flattening step for flattening the substrate held by the chuck table by grinding it with grinding means 10; a bonding step for bonding the workpiece onto the flattened substrate; a workpiece holding step for holding, after carrying out the bonding step, the substrate side of the workpiece by the same chuck table used in the substrate flattening step; and a workpiece grinding step for grinding to flatten the workpiece, which is held by the chuck table in the workpiece holding step, with the same grinding means used in the substrate flattening step.