A screw pump includes a casing including a main accommodating bore and plural sub accommodating bores, a main rotor adapted to be rotatably accommodated in the main accommodating bore, and plural sub rotors adapted to be respectively rotatably accommodated in the sub accommodating bores. Each sub rotor is adapted to engage the main rotor. A cross sectional shape of a flank surface of the main rotor, which is taken in a direction perpendicular to an axial direction of the main rotor, is formed along an epitrochoid traced by an edge of the sub rotor not to include an undercut shape. Further, a cross sectional shape of a flank surface of each sub rotor, which is taken in a direction perpendicular to an axial direction of the sub rotor, is formed along an epitrochoid traced by an edge of the main rotor not to include an undercut shape.