A DC-DC converter includes an insulating substrate with an inductor provided on the top surface thereof, a switching control IC provided therein, and a ground electrode pattern provided on the bottom surface thereof. The ground electrode pattern includes a first pattern and a second pattern separated from each other and a bridge pattern that connects the first and second patterns to each other. A capacitor and the switching control IC is connected to each of the first and second patterns. The bridge pattern faces the inductor and has a smaller width than that of the first and second patterns.