A power transmission device comprising: an annular transmission unit having an annular first magnetic core and a transmission coil; and an annular reception unit having an annular second magnetic core and a reception coil, wherein the transmission unit and the reception unit are arranged to face each other with a virtual reference plane interposed therebetween, the power transmission device transmits power from the transmission unit to the reception unit, the transmission unit and the reception unit are relatively rotatable about a rotation axis which is a virtual axis orthogonal to the virtual reference plane and which is passing through the center of and a cavity inside the transmission unit and the center of and a cavity inside the reception unit, and the first magnetic core has a structure divided into a plurality of first split cores in a circumferential direction, and the second magnetic core has a structure divided into a plurality of second split cores in the circumferential direction, wherein the transmission unit includes a first holder member made of resin and holding the first magnetic core, the first holder member comprising a first annular portion having an annular shape arranged along one surface of the first magnetic core facing in an axial direction along the rotation axis and a plurality of first upright wall portions each of which being disposed at each of a plurality of locations in the circumferential direction along an outer peripheral surface of the first magnetic core, each of the plurality of first upright wall portions including a first engagement claw portion that engages with a surface of the first magnetic core opposite to the one surface of the first magnetic core, and wherein the reception unit includes a second holder member made of resin and holding the second magnetic core, the second holder member comprising a second annular portion having an annular shape arranged along one surface of the second magnetic core facing in the axial direction and a plurality of second upright wall portions each of which being disposed at each of a plurality of locations in the circumferential direction along an outer peripheral surface of the second magnetic core, each of the plurality of second upright wall portions including a second engagement claw portion that engages with a surface of the second magnetic core opposite to the one surface of the second magnetic core. an annular transmission unit having an annular first magnetic core and a transmission coil; and an annular reception unit having an annular second magnetic core and a reception coil, wherein the transmission unit and the reception unit are arranged to face each other with a virtual reference plane interposed therebetween, the power transmission device transmits power from the transmission unit to the reception unit, the transmission unit and the reception unit are relatively rotatable about a rotation axis which is a virtual axis orthogonal to the virtual reference plane and which is passing through the center of and a cavity inside the transmission unit and the center of and a cavity inside the reception unit, and the first magnetic core has a structure divided into a plurality of first split cores in a circumferential direction, and the second magnetic core has a structure divided into a plurality of second split cores in the circumferential direction, wherein the transmission unit includes a first holder member made of resin and holding the first magnetic core, the first holder member comprising a first annular portion having an annular shape arranged along one surface of the first magnetic core facing in an axial direction along the rotation axis and a plurality of first upright wall portions each of which being disposed at each of a plurality of locations in the circumferential direction along an outer peripheral surface of the first magnetic core, each of the plurality of first upright wall portions including a first engagement claw portion that engages with a surface of the first magnetic core opposite to the one surface of the first magnetic core, and wherein the reception unit includes a second holder member made of resin and holding the second magnetic core, the second holder member comprising a second annular portion having an annular shape arranged along one surface of the second magnetic core facing in the axial direction and a plurality of second upright wall portions each of which being disposed at each of a plurality of locations in the circumferential direction along an outer peripheral surface of the second magnetic core, each of the plurality of second upright wall portions including a second engagement claw portion that engages with a surface of the second magnetic core opposite to the one surface of the second magnetic core.