A groove (Da) extending in a Y-axis direction of a workpiece (D) is finish machined with an offset tool (T) that has a cross-section with a shape (L3) offset in a reducing direction by only a predetermined offset amount relative to a finished shape (L1) of the groove (Da) and that rotates around an axis of rotation parallel to a Z axis. Each time an incising step in which the offset tool (T) is relatively moved within the X-Z plane only by a predetermined distance on the circumference of an imaginary circle (C) having an initial position as its center and the offset amount (o) as its radius is carried out, a cutting step in which the entire length of the groove (Da) is cut while relatively moving the offset tool (T) in the Y-axis direction is carried out repeatedly. This enables the load imposed on the offset tool (T) to be suppressed compared with the case of a forming tool, which cuts a wide region of the groove (Da) of the workpiece (D) all at once, thus providing a method of machining a workpiece with an offset tool (T), the method allowing the durability of the offset tool (T) to be enhanced and the finishing precision of a machined face to be increased.