In a sprocket for a roller chain, the tooth form is such that the distance between the tooth center line and a part of the front tooth surface, at a location on the tooth head side of the pitch line, is greater than the distance between the tooth center line and the front surface of the tooth along the pitch line. The diameter of the tooth gap bottom circle is smaller than the diameter of a tooth gap bottom circle in a sprocket having an ISO tooth form for the same chain.