An oil pump rotor for use in an oil pump with a casing having an intake port for drawing in a fluid and a discharge port for discharging the fluid, and transferring a fluid by drawing in and discharging said fluid as volumes of cells formed between tooth surfaces of two rotors (10, 20) change when said two rotors (10, 20) rotate while being engaged with each other, comprising: an inner rotor having n (n is a natural number) external teeth, said inner rotor exhibiting a tooth tip shape established by an epicycloid curve that is generated by a first outer rolling circle Di externally tangent to and rolling on a base circle bi of said inner rotor without slipping and a tooth groove shape established by a hypocycloid curve that is generated by a first inner rolling circle di internally tangent to and rolling within said base circle bi without slipping; and an outer rotor having n+1 internal teeth, said outer rotor exhibiting a tooth groove shape established by an epicycloid curve that is generated by a second outer rolling circle Do externally tangent to and rolling on a base circle bo of said outer rotor without slipping and a tooth tip shape established by a hypocycloid curve that is generated by a second inner rolling circle do internally tangent to and rolling within said base circle bo without slipping; wherein when a diameter of said base circle bi of said inner rotor is Φ bi; a diameter of said first outer rolling circle Di is Φ Di; a diameter of said first inner rolling circle di is Φ di; a diameter of said base circle bo of said outer rotor is Φ bo; a diameter of said second outer rolling circle Do is Φ Do; a diameter of a second inner rolling circle do is Φ do; and an eccentricity amount between said inner rotor and said outer rotor is e, Φ bi = n·(Φ Di + Φ di) and Φ bo = (n + 1)· (Φ Do + Φ do) hold; either Φ Di + Φ di = 2e or Φ Do + Φ do = 2e holds; and Φ Do>Φ Di, Φ di>Φ do and (Φ Di + Φ di)<(Φ Do+ Φ do) hold, and characterized in that when a clearance between said inner rotor and said outer rotor is t, 0.3 ≤ ((Φ Do + Φ do) - (Φ Di + Φ di)) · (n + 1)/t≤0.6 holds, provided that Φ Di + Φ di = 2e; or 0.3 ≤ ((Φ Do + Φ do) - (Φ Di + Φ di))·n/t≤0.6 holds, provided that Φ Do + Φ do = 2e. an inner rotor having n (n is a natural number) external teeth, said inner rotor exhibiting a tooth tip shape established by an epicycloid curve that is generated by a first outer rolling circle Di externally tangent to and rolling on a base circle bi of said inner rotor without slipping and a tooth groove shape established by a hypocycloid curve that is generated by a first inner rolling circle di internally tangent to and rolling within said base circle bi without slipping; and an outer rotor having n+1 internal teeth, said outer rotor exhibiting a tooth groove shape established by an epicycloid curve that is generated by a second outer rolling circle Do externally tangent to and rolling on a base circle bo of said outer rotor without slipping and a tooth tip shape established by a hypocycloid curve that is generated by a second inner rolling circle do internally tangent to and rolling within said base circle bo without slipping; wherein when a diameter of said base circle bi of said inner rotor is Φ bi; a diameter of said first outer rolling circle Di is Φ Di; a diameter of said first inner rolling circle di is Φ di; a diameter of said base circle bo of said outer rotor is Φ bo; a diameter of said second outer rolling circle Do is Φ Do; a diameter of a second inner rolling circle do is Φ do; and an eccentricity amount between said inner rotor and said outer rotor is e, Φ bi = n·(Φ Di + Φ di) and Φ bo = (n + 1)· (Φ Do + Φ do) hold; either Φ Di + Φ di = 2e or Φ Do + Φ do = 2e holds; and Φ Do>Φ Di, Φ di>Φ do and (Φ Di + Φ di)<(Φ Do+ Φ do) hold, and characterized in that when a clearance between said inner rotor and said outer rotor is t, 0.3 ≤ ((Φ Do + Φ do) - (Φ Di + Φ di)) · (n + 1)/t≤0.6 holds, provided that Φ Di + Φ di = 2e; or 0.3 ≤ ((Φ Do + Φ do) - (Φ Di + Φ di))·n/t≤0.6 holds, provided that Φ Do + Φ do = 2e.