PURPOSE: To meet intermeshing condition with no dead space by making tooth profiles of an outer rotor and an inner rotor circular arcs on a parts of top portions and bottom portions for meshing each other, and connecting together circular arcs of neighboring top portion and bottom portion of each with a continuous curve.

CONSTITUTION: An inner rotor I and an outer rotor II are contacted together at a top circular arc b-a-b and a bottom circular arc B-A-B in the second and the third quadrant. The rotor I is thought to rotate in clockwise around the center O' under this condition. The rotor II is given with rotary motion being inversely proportional to gear ratio by a rotation transmitting mechanism, and a circular arc part ED of the rotor II in the third quadrant X1OY2 is contacted nearly slidingly with a curve c-d of the rotor I, and a curve D-E-D and a circular arc rotate so as to slidingly contact with curves d-e-d and d-c, respectively in order. Further, (A-B) of the corresponded rotor II is contacted with (a-b) of the rotor I, and curves (C-D) and (D-E) contacting with (c-d) and (d-e) slidingly, in another quadrant.