A gear pump comprising a first gear and a second gear provided to freely rotate about mutually parallel rotary shafts (15, 25) and meshed to each other, and a casing having a disposal space for holding said first gear and said second gear such that respective tooth crests and side faces thereof slide on wall surface portions (41, 42) of said casing, with a suction chamber into which a fluid is suctioned and a discharge chamber from which said fluid is discharged as said first gear and said second gear rotate in said casing, wherein - said wall surface portion of said casing has a first partition surface on which said tooth crests of said first gear slide, a second partition surface on which said tooth crests of said second gear slide, a suction side inner peripheral surface that connects said first partition surface and said second partition surface on said suction chamber side, and a discharge side inner peripheral surface that connects said first partition surface and said second partition surface on said discharge chamber side, - fluid communication portions are provided in connecting portions between said respective partition surfaces (41, 42) and said discharge side inner peripheral surface to cause tooth spaces (11, 21) surrounded by said wall surface portion and teeth (12, 22) of said gears (10, 15) positioned near said partition surface side (41, 42) of said connecting portions to communicate gradually with said discharge chamber as said gears (10, 20) rotate, - said casing comprises a first side face on which one (13, 23) of said respective side faces of said first gear and said second gear slides, and a second side face on which the other one (14, 24) of said respective side faces slides, and - said fluid communication portions are formed, in said connecting portions, from tapered surfaces that extend between said respective partition surfaces (41, 42) and said discharge side inner peripheral surface and incline from said first side face side to said second side face side, characterized in that - the inclined surface of the tapered portion is formed as a chamfered portion having a substantially triangular shape, in which three points P2, P3 and P4 serve as vertices, - when an internal vertex of a substantially triangular pyramid T of the pump case is cut in planes passing through the points P2, P3, P4, - an intersection between the arc defining the driven side partition surface and a line defining the discharge side inner peripheral surface in the base end surface of the pump case is set as a point P1, - the point P2 is obtained by moving in an inverse rotation direction of the driven gear along the arc from the point P1 by an angle θ about a rotary axis of the driven gear, - the point P3 is located on the intersection between a tangent of the point on the arc and the line, and - the point P4 is located on the intersection of the driven side partition surface, the discharge side inner peripheral surface and the sliding partition surface. - said wall surface portion of said casing has a first partition surface on which said tooth crests of said first gear slide, a second partition surface on which said tooth crests of said second gear slide, a suction side inner peripheral surface that connects said first partition surface and said second partition surface on said suction chamber side, and a discharge side inner peripheral surface that connects said first partition surface and said second partition surface on said discharge chamber side, - fluid communication portions are provided in connecting portions between said respective partition surfaces (41, 42) and said discharge side inner peripheral surface to cause tooth spaces (11, 21) surrounded by said wall surface portion and teeth (12, 22) of said gears (10, 15) positioned near said partition surface side (41, 42) of said connecting portions to communicate gradually with said discharge chamber as said gears (10, 20) rotate, - said casing comprises a first side face on which one (13, 23) of said respective side faces of said first gear and said second gear slides, and a second side face on which the other one (14, 24) of said respective side faces slides, and - said fluid communication portions are formed, in said connecting portions, from tapered surfaces that extend between said respective partition surfaces (41, 42) and said discharge side inner peripheral surface and incline from said first side face side to said second side face side, - the inclined surface of the tapered portion is formed as a chamfered portion having a substantially triangular shape, in which three points P2, P3 and P4 serve as vertices, - when an internal vertex of a substantially triangular pyramid T of the pump case is cut in planes passing through the points P2, P3, P4, - an intersection between the arc defining the driven side partition surface and a line defining the discharge side inner peripheral surface in the base end surface of the pump case is set as a point P1, - the point P2 is obtained by moving in an inverse rotation direction of the driven gear along the arc from the point P1 by an angle θ about a rotary axis of the driven gear, - the point P3 is located on the intersection between a tangent of the point on the arc and the line, and - the point P4 is located on the intersection of the driven side partition surface, the discharge side inner peripheral surface and the sliding partition surface.