A screw rotor for use in a screw pump that pumps fluid by rotation of a pair of screw rotors (13, 14) engaged with each other in a rotor housing, the screw rotor (13, 14) including a multiple-thread portion (26, 34) for pump suction side and a single-thread portion (29, 37) for pump discharge side, characterized in that the single-thread portion (29, 37) has a tooth profile (G11, G12) in an imaginary plane that is perpendicular to the rotation axis (151, 161) of the screw rotor (13, 14), the tooth profile (G11, G12) of the single-thread portion (29, 37) including: a tooth top portion (A1B1, A2B2) extending circularly around the rotation axis (151, 161); a tooth bottom portion (C1D1, C2D2) extending circularly around the rotation axis (151, 161), the radius of the tooth bottom portion (C1D1, C2D2) being smaller than the radius of the tooth top portion (A1B1, A2B2); a first curved portion (A1C1, A2C2) formed by a trochoidal curve, the first curved portion (A1C1, A2C2) connecting one end (A1, A2) of the tooth top portion (A1B1, A2B2) to one end (C1, C2) of the tooth bottom portion (C1 D1, C2D2); and a second curved portion (B1D1, B2D2) connecting the other end (B1, B2) of the tooth top portion (A1B1, A2B2) to the other end (D1, D2) of the tooth bottom portion (C1D1, C2D2), the multiple-thread portion (26, 34) has a tooth profile (G21, G22) in an imaginary plane that is perpendicular to the rotation axis (151, 161) of the screw rotor (13, 14), the tooth profile (G21, G22) of the multiple-thread portion (26, 34) including: a tooth top portion (42A, 42B) extending circularly around the rotation axis (151, 161); a tooth bottom portion (43A, 43B) extending circularly around the rotation axis (151, 161), the radius of the tooth bottom portion (43A, 43B) being smaller than the radius of the tooth top portion (42A, 42B); and a third curved portion (46A, 46B) formed by a trochoidal curve, the third curved portion (46A, 46B) connecting one end (422A, 422B) of the tooth top portion (42A, 42B) to one end of the tooth bottom portion (43A, 43B), the screw rotor (13, 14) is formed so that the tooth profile (G21, G22) of the multiple-thread portion (26, 34) is connected to the tooth profile (G11, G12) of the single-thread portion (29, 37) through a boundary plane (38, 39) that is perpendicular to the rotation axis (151, 161) of the screw rotor (13, 14), and the first curved portion (A1C1, A2C2) satisfies the following condition or: the first curved portion (A1C1, A2C2) coincides with the third curved portion (46A, 46B) in the boundary plane (38, 39); the first curved portion (A1C1, A2C2) is angularly spaced from the third curved portion (46A, 46B) by an angular difference αo in the direction opposite to the rotation direction (W, Z) of the screw rotor (13, 14), and the angular difference αo < θ is satisfied, where θ is a tooth top portion angle between the line (L11, L21) extending from the one end (422A, 422B) of the tooth top portion (42A, 42B) of the multiple-thread portion (26, 34) to the rotation axis (151, 161) of the screw rotor (13, 14) and the line (L12, L22) extending from the other end (421A, 421B) of the tooth top portion (42A, 42B) of the multiple-thread portion (26, 34) to the rotation axis (151, 161) of the screw rotor (13, 14) in the boundary plane (38. 39). a tooth top portion (A1B1, A2B2) extending circularly around the rotation axis (151, 161); a tooth bottom portion (C1D1, C2D2) extending circularly around the rotation axis (151, 161), the radius of the tooth bottom portion (C1D1, C2D2) being smaller than the radius of the tooth top portion (A1B1, A2B2); a first curved portion (A1C1, A2C2) formed by a trochoidal curve, the first curved portion (A1C1, A2C2) connecting one end (A1, A2) of the tooth top portion (A1B1, A2B2) to one end (C1, C2) of the tooth bottom portion (C1 D1, C2D2); and a second curved portion (B1D1, B2D2) connecting the other end (B1, B2) of the tooth top portion (A1B1, A2B2) to the other end (D1, D2) of the tooth bottom portion (C1D1, C2D2), a tooth top portion (42A, 42B) extending circularly around the rotation axis (151, 161); a tooth bottom portion (43A, 43B) extending circularly around the rotation axis (151, 161), the radius of the tooth bottom portion (43A, 43B) being smaller than the radius of the tooth top portion (42A, 42B); and a third curved portion (46A, 46B) formed by a trochoidal curve, the third curved portion (46A, 46B) connecting one end (422A, 422B) of the tooth top portion (42A, 42B) to one end of the tooth bottom portion (43A, 43B), the first curved portion (A1C1, A2C2) coincides with the third curved portion (46A, 46B) in the boundary plane (38, 39); the first curved portion (A1C1, A2C2) is angularly spaced from the third curved portion (46A, 46B) by an angular difference αo in the direction opposite to the rotation direction (W, Z) of the screw rotor (13, 14), and the angular difference αo < θ is satisfied, where θ is a tooth top portion angle between the line (L11, L21) extending from the one end (422A, 422B) of the tooth top portion (42A, 42B) of the multiple-thread portion (26, 34) to the rotation axis (151, 161) of the screw rotor (13, 14) and the line (L12, L22) extending from the other end (421A, 421B) of the tooth top portion (42A, 42B) of the multiple-thread portion (26, 34) to the rotation axis (151, 161) of the screw rotor (13, 14) in the boundary plane (38. 39).