There is provided a power transmission control device included in a power transmission device in a contactless power transmission system that transmits power from the power transmission device to a power receiving device by electromagnetically coupling a primary coil to a secondary coil to supply the power to a load of the power receiving device. The power transmission control device includes a controller controlling the power transmission control device, a host interface communicating with a power transmission-side host, and a register section accessible from the power transmission-side host via the host interface. The controller shifts into a communication mode that executes communication between the power transmission-side host and a power receiving-side host, when the power transmission-side host writes, via the host interface, a communication request command that requests the communication between the hosts in the register section. Then, the controller transmits the communication request command to the power receiving device.