A cooling fin structure used in a cooler for an electric device, comprising a plurality of pin fins arranged in a zigzag form in a coolant passage through which a coolant flows such that a plurality of pin fins (71; 71A) arranged in a line on a centerline in a direction of flow of the coolant and a plurality of pin fins (71; 71B) located adjacent to the plurality of pin fins (71; 71A) and arranged in a line on a centerline in the direction of flow of the coolant are shifted from each other in a direction orthogonal to the direction of flow of the coolant, characterized in that: each of the pin fins has a circular portion having a circular cross-section, and curved portions provided contiguously on the upstream side and downstream side of the circular portion as viewed in a direction of flow of the coolant; the curved portions have an outer peripheral surface that is formed along a circumference having a center at a center point of the circular portion of an adjacent one of the pin fins that is located adjacent to the pin fin having the curved portions, in an oblique direction relative to the direction of flow of the coolant; and each of the curved portions has two opposite curved faces formed contiguously with outer peripheral surfaces of the circular portion and each of the two opposite curved faces is provided by the outer peripheral surface formed along the circumference, wherein, when a first pitch (Py) between the pin fins located adjacent to each other in an oblique direction relative to the direction of flow of the coolant, as measured in the direction orthogonal to the direction of flow of the coolant, is equal to R+1/2xCL, and a second pitch (Px) between the pin fins located adjacent to each other in the oblique direction, as measured in the direction of flow of the coolant, is equal to 3 1/2 (R+1/2×CL), where R is a radius of the circular portion, and CL is a clearance between the pin fins located adjacent to each other in the direction orthogonal to the direction of flow of the coolant, the outer peripheral surface is shaped along the circumference having a radius of R+CL. each of the pin fins has a circular portion having a circular cross-section, and curved portions provided contiguously on the upstream side and downstream side of the circular portion as viewed in a direction of flow of the coolant; the curved portions have an outer peripheral surface that is formed along a circumference having a center at a center point of the circular portion of an adjacent one of the pin fins that is located adjacent to the pin fin having the curved portions, in an oblique direction relative to the direction of flow of the coolant; and each of the curved portions has two opposite curved faces formed contiguously with outer peripheral surfaces of the circular portion and each of the two opposite curved faces is provided by the outer peripheral surface formed along the circumference, wherein, when a first pitch (Py) between the pin fins located adjacent to each other in an oblique direction relative to the direction of flow of the coolant, as measured in the direction orthogonal to the direction of flow of the coolant, is equal to R+1/2xCL, and a second pitch (Px) between the pin fins located adjacent to each other in the oblique direction, as measured in the direction of flow of the coolant, is equal to 3 1/2 (R+1/2×CL), where R is a radius of the circular portion, and CL is a clearance between the pin fins located adjacent to each other in the direction orthogonal to the direction of flow of the coolant, the outer peripheral surface is shaped along the circumference having a radius of R+CL.