On 15 January 2022, the submarine volcano on the southwest Pacific island of Tonga violently erupted. Thus far, the ionospheric oscillation features caused by the volcanic eruption have not been identified. Here, the field-aligned electron density irregularities in the ionosphere detected by Super Dual Auroral Radar Network (SuperDARN) radars are employed as tracers to analyse ionospheric oscillation in the Northern Hemisphere caused by the volcanic eruption in Tonga. Due to the magnetic field conjugate effect, the ionospheric oscillations were observed much earlier than expected, and the maximum negative line-of-sight (LOS) velocity of the ionospheric oscillation exceeded 100 m/s in the F layer. After the surface air pressure wave arrived, the maximum LOS velocity in the E layer approached 150 m/s. The ionosphere also experienced a maximum upwards displacement of 100 km. This work provides a new perspective for understanding the strong ionospheric oscillation caused by geological hazards observed on Earth.