In a resonant device, a third plane electrode is located between a first plane electrode and a second plane electrode. A first via electrode connects the first plane electrode and the third plane electrode. A second via electrode, a third via electrode, and a fourth via electrode connect the first plane electrode and the second plane electrode. When the third plane electrode is seen in plan view in a normal direction of the first plane electrode, the third plane electrode includes a first projecting portion overlapping with a first area between the second via electrode and the third via electrode, and a second projecting portion overlapping with a second area between the third via electrode and the fourth via electrode.