A detection device for detecting a state of electric power has a first current transformer that measures a first current on a first power line side of a connecting point between a first power line derived from a commercial power supply and a second power line derived from a power generation device which supplies electric power equal in frequency to the commercial power supply, a second current transformer that measures a second current on a second power line side of the connecting point, a first calculation unit that calculates a decision value based on a product of a measured value of the first current and a measured value of the second current, and a detection unit that detects a power flow direction of electric power of the first power line based on the decision value.