A power transmission device (1, 200) comprising: a housing (10, 12, 14, 15, 210, 212, 214, 215) arranged to form a mounting space by connecting a clutch housing (14, 214), a clutch connecting portion (12, 212), a clutch hub (10, 210) and a disk housing (15, 215) to each other; a power transmission hub (96, 260) selectively and operably connected to the housing (10, 12, 14, 15, 210, 212, 214, 215); a plurality of first friction disks (91, 250) splined to an interior circumference of the disk housing (15, 215); a plurality of second friction disks (92, 252) splined to an exterior circumference of the power transmission hub (96, 260) and disposed alternately with the first friction disks (91, 250); a piston module including two or more pistons (32, 70), each of which has a corresponding piston chamber (38, 48), and selectively and frictionally coupling the first friction disks (91, 250) with the second friction disks (92, 252) by an operating hydraulic pressure supplied to the piston chambers (38, 48); and at least one return spring (60, 240) or at least one separating spring supplying a spring load counteracting against an axial force generated by the operating hydraulic pressure, and disposed at an inside or an outside of the piston module, wherein at least one pushing portion (78, 80) for directly applying the axial force onto the first friction disk (91, 250) or the second friction disk (92, 252) is formed at at least one of the two or more pistons (32, 70) included in the piston module, and wherein a ratio (u) of an average radius (RAm) of the at least one pushing portion (78, 80) to an average radius (RPm) of the two or more pistons (32, 70) is within a range of 0.70 < u < 2.0, the spring load F s (kgf) of the at least one return spring (60, 240) or the at least one separating spring acting on the piston module being within a range of 0.45 bar ≤ F S A ≤ P − 1.2 A c A × P − 1 bar, where "A" (cm 2) indicates a total piston area of the two or more pistons, "P" (kgf/cm 2) indicates the operating hydraulic pressure, and "A c " (cm 2) indicates a pressure chamber area formed by the housing (10, 12, 14, 15, 210, 212, 214, 215), characterized in that friction material is attached to any one surface or both surfaces of each first friction disk (91, 250) or each second friction disk (92, 252), and wherein a ratio (v) of an average radius (RFm) of the friction material to the average radius (RPm) of the two or more pistons (32, 70) is within a range of 0.80 < v < 1.25. a housing (10, 12, 14, 15, 210, 212, 214, 215) arranged to form a mounting space by connecting a clutch housing (14, 214), a clutch connecting portion (12, 212), a clutch hub (10, 210) and a disk housing (15, 215) to each other; a power transmission hub (96, 260) selectively and operably connected to the housing (10, 12, 14, 15, 210, 212, 214, 215); a plurality of first friction disks (91, 250) splined to an interior circumference of the disk housing (15, 215); a plurality of second friction disks (92, 252) splined to an exterior circumference of the power transmission hub (96, 260) and disposed alternately with the first friction disks (91, 250); a piston module including two or more pistons (32, 70), each of which has a corresponding piston chamber (38, 48), and selectively and frictionally coupling the first friction disks (91, 250) with the second friction disks (92, 252) by an operating hydraulic pressure supplied to the piston chambers (38, 48); and at least one return spring (60, 240) or at least one separating spring supplying a spring load counteracting against an axial force generated by the operating hydraulic pressure, and disposed at an inside or an outside of the piston module, wherein at least one pushing portion (78, 80) for directly applying the axial force onto the first friction disk (91, 250) or the second friction disk (92, 252) is formed at at least one of the two or more pistons (32, 70) included in the piston module, and wherein a ratio (u) of an average radius (RAm) of the at least one pushing portion (78, 80) to an average radius (RPm) of the two or more pistons (32, 70) is within a range of 0.70 < u < 2.0, the spring load F s (kgf) of the at least one return spring (60, 240) or the at least one separating spring acting on the piston module being within a range of 0.45 bar ≤ F S A ≤ P − 1.2 A c A × P − 1 bar, where "A" (cm 2) indicates a total piston area of the two or more pistons, "P" (kgf/cm 2) indicates the operating hydraulic pressure, and "A c " (cm 2) indicates a pressure chamber area formed by the housing (10, 12, 14, 15, 210, 212, 214, 215), friction material is attached to any one surface or both surfaces of each first friction disk (91, 250) or each second friction disk (92, 252), and wherein a ratio (v) of an average radius (RFm) of the friction material to the average radius (RPm) of the two or more pistons (32, 70) is within a range of 0.80 < v < 1.25.