A power transmission device adapted to transmit electric power to a power reception device in a contactless manner, comprising: a power transmission coil unit; an electrical device connected to said power transmission coil unit; and an enclosure configured to store said power transmission coil unit and said electrical device, said enclosure including an installation wall located on an installation surface, an outer wall provided to cover said installation wall from a power transmission surface opposite to the installation surface and leaving space in said enclosure, a partitioning wall configured to partition said space, and a cover portion configured to close an opening formed in the power transmission surface of said outer wall and to allow transmission of electric power by said power transmission coil unit, said partitioning wall being provided to partition said space in said enclosure into a coil-unit storing portion in which said cover portion is located and said power transmission coil unit is stored, and an electrical-device storing portion in which said electrical device is stored, said outer wall and said partitioning wall being made of metal, said outer wall having a main surface wall having said opening formed therein and provided on said power transmission surface, and a sidewall (630, 662) located on the side, between the installation wall and the main surface wall, said power transmission coil unit including a power transmission coil, said power transmission coil having a hole formed therein and being arranged such that a winding axis extends through said cover portion and said installation wall, said electrical device at least including a filter device portion (716, 717, 720, 721), an AC/DC device portion and a DC/RF device portion, and at least one of said filter device portion (716, 717, 720, 721), said AC/DC device portion and said DC/RF device portion being arranged at a position distant from a region located between said hole in said power transmission coil and said installation wall. a power transmission coil unit; an electrical device connected to said power transmission coil unit; and an enclosure configured to store said power transmission coil unit and said electrical device, said enclosure including an installation wall located on an installation surface, an outer wall provided to cover said installation wall from a power transmission surface opposite to the installation surface and leaving space in said enclosure, a partitioning wall configured to partition said space, and a cover portion configured to close an opening formed in the power transmission surface of said outer wall and to allow transmission of electric power by said power transmission coil unit, an installation wall located on an installation surface, an outer wall provided to cover said installation wall from a power transmission surface opposite to the installation surface and leaving space in said enclosure, a partitioning wall configured to partition said space, and a cover portion configured to close an opening formed in the power transmission surface of said outer wall and to allow transmission of electric power by said power transmission coil unit, said partitioning wall being provided to partition said space in said enclosure into a coil-unit storing portion in which said cover portion is located and said power transmission coil unit is stored, and an electrical-device storing portion in which said electrical device is stored, said outer wall and said partitioning wall being made of metal, said outer wall having a main surface wall having said opening formed therein and provided on said power transmission surface, and a sidewall (630, 662) located on the side, between the installation wall and the main surface wall, said power transmission coil unit including a power transmission coil, said power transmission coil having a hole formed therein and being arranged such that a winding axis extends through said cover portion and said installation wall, said electrical device at least including a filter device portion (716, 717, 720, 721), an AC/DC device portion and a DC/RF device portion, and at least one of said filter device portion (716, 717, 720, 721), said AC/DC device portion and said DC/RF device portion being arranged at a position distant from a region located between said hole in said power transmission coil and said installation wall.