Power transmission device and electric power transmission system

A power transmission device that wirelessly transmits electric power to a power reception device, the power transmission device comprising:

• a power transmission coil configured by winding a conductive wire;
• and
• a foreign object detection device detecting a foreign object, wherein
• the foreign object detection device includes:
• sensor coils placed to cover the power transmission coil;
• and
• a detector executing determination processing of determining existence of the foreign object on each of the sensor coils, based on a comparison result between a comparison target value based on output voltage output from one sensor coil of the sensor coils and a threshold value set to the one sensor coil, and
• the detector executes threshold value change processing of changing the threshold value set to the one sensor coil on each of the sensor coils, based on induced voltage induced in the one sensor coil by magnetic flux generated by the power transmission coil.