A DC-DC converter with low power consumption and high power conversion efficiency is provided. The DC-DC converter includes a first transistor and a control circuit. The control circuit includes an operational amplifier generating a signal that controls switching of the first transistor, a bias circuit generating a bias potential supplied to the operational amplifier, and a holding circuit holding the bias potential. The holding circuit includes a second transistor and a capacitor to which the bias potential is supplied. The first transistor and the second transistor include a first oxide semiconductor film and a second oxide semiconductor film, respectively. The first oxide semiconductor film and the second oxide semiconductor film each contain In, M (M is Ga, Y, Zr, La, Ce, or Nd), and Zn. The atomic ratio of In to M in the first oxide semiconductor film is higher than that in the second oxide semiconductor film.