An ultrasonic trimming method is composed of steps, driving a cutter blade having a flat plate shape and supported by an arm at an end of an articulated robot via an ultrasonic oscillator, the cutter blade being driven by the arm while the cutter blade is ultrasonically vibrated; cutting a workpiece secured by a workpiece securing portion; and during, operation for cutting, moving the cutter blade held attached to the articulated robot to the position at where the cutting edge is brought into contact with the grindstone, maintaining the cutter blades attitude such that a plane containing the cutting edge thereof contacts the grindstone, and grinding the cutter blade by pressing the cutter blade against a grinding member by the arm while the cutter blade is ultrasonically vibrated, the grinding member being disposed within a movable range of the cutter blade driven by the articulated robot.