The number of teeth is set to 4n+2 (n is a natural number), both the ends on a
long axis are set to be teeth grooves, both the ends on a
short axis are set to be tips, a
pitch curve is set to be, for example, an
ellipse, and tooth marks 31-37, 41-47 are set to be basic tooth mark curves. With the tooth mark curve, the engagement teeth surface is set to be an
involute curve, a non-engagement
tooth surface is set to be a
cycloid curve, and the tool
pressure angle of each tooth mark is set by an
undercut limit and a point limit. The non-circular gears 30, 50 bury the recess between two tooth marks, holding the
tooth space located at both the ends on the
long axis, based on the basic tooth mark curve, and have a shape, where the tooth section including the tip at both the ends on the
short axis are scraped off, and the number of teeth completed is finally set to 4n-2. Accordingly, it is able to obtain a non-circular gear that reduces the change in an engagement
pressure angle, is advantageous for setting the tool
pressure angle of an engagement teeth surface, can reduce the number of teeth without forming an overhang section, and can fully secure the sealing performance with the inner wall of a casing to be installed.