To provide a gear having an arcuate tooth shape which is simple and easy to machine, and to provide a circumscribed gear type pump and a gear transmission device each comprising a plurality of gears including the gears.

The tooth size of the gear can be represented by an arc radius R forming the tooth shape of the tooth end face. The tooth shape of the tooth end face is formed by an arc of the radius R around a reference pitch line. The tooth shape of the tooth end face is formed by arcs of three circumscribed circles circumscribed on two circles of the radius R forming the tooth shape of the tooth end face adjacent to a tooth bottom circle. The gear has the tooth shape where the arc having the tooth shape of the tooth end face is connected to the arc having the tooth shape of a dedendum face in contacting relation. The circumscribed gear type pump and the gear transmission device each comprise the plurality of gears including the gears each using the tooth shape.