A power transmission device includes an inverter, an oscillator, a foreign substance detector, and a power transmission control circuitry. The power transmission control circuitry causes the foreign substance detector to perform a series of multiple processes and determine whether a foreign substance is present before a transmission of first AC power starts, and then causes the inverter to start the transmission of the first AC power. After the transmission starts, a detection period in which foreign substance detecting is performed and a power transmission period in which transmission of the first AC power is performed are repeated. The series of multiple processes is divided and performed in the multiple repeated detecting periods. The foreign substance detector is caused to divide and perform the series of multiple processes using the detecting periods and determine whether a foreign substance is present.