In an ignition device for use in an internal combustion engine in which a spark plug and an ignition coil are integrated with each other and mounted in a cylinder head, a case is made of an electrical conductive material and one end portion of a secondary winding of the ignition coil is electrically connected to a center electrode of the spark plug while the other end portion of the secondary winding and an earth electrode of the spark plug are electrically connected to the case. Thus, the low-voltage side of the secondary winding and the earth electrode are electrically connected to each other through the case. This eliminates the need for a connector terminal and a wire harness for the electrical connection of the low-voltage side of the secondary winding to the internal combustion engine.