1. A machining data generator for generating machining data for shaping an external shape of a piston, said machining data generator comprising: a spreadsheet software unit that describes noncircular part shape data defined by a longitudinal coordinate on an axis of a workpiece, an angular coordinate corresponding to the longitudinal coordinate, and a radial coordinate corresponding to the angular coordinate in a form of matrix data in a machining data sheet, and describes shape-data-describing-are-specifying data that specifies a cell region in which the matrix data is entered, and machining condition data that specifies machining conditions for shaping the noncircular part of the piston, after directives in the machining data sheet; a noncircular part shape data fetching unit that recognizes the cell region specified by the shape-data-describing-area-specifying data in reading the machining data sheet and fetching the noncircular part shape data; and a NC machining data calculating unit that recognizes the machining condition data described in the machining data sheet and that calculates NC machining data on the basis of the recognized machining condition data and the noncircular part shape data fetched by the noncircular part shape data fetching unit.