A coupler includes first and second electrodes and a nonlinear element including a Josephson junction. The first and second electrodes are spaced apart from a ground plane surrounding peripheries thereof and arranged opposed to each other. The first/second electrode includes two opposing portions extended toward first and second qubits/third and fourth qubits. At least either one of a gap between the first electrode and the ground plane facing the first electrode and a gap between the second electrode and the ground plane facing the second electrode, includes a gap width of at least a value of a same extent as or a fraction of a size of the first electrode or the second electrode.