A 72-year-old man with stage IV hypopharyngeal cancer, who had been treated for three months with combination chemotherapy, was referred to our cardiology department for evaluation of transient palpitation. Combination therapy with cetuximab, cisplatin, and 5-fluorouracil per cycle had been administered intravenously for five cycles every three weeks for three months. After the admission due to slight palpitation and severe hypomagnesemia (Mg = 0.6 mmol/L), monitor ECG showed supraventricular tachycardia (SVT), which was incessantly sustained and ceased every few minutes. 12-lead ECG obtained during tachycardia demonstrated long RP' narrow QRS tachycardia. SVT was initially considered to be related to severe hypomagnesemia. However, it still occurred even after normalization of serum magnesium level. As the SVT was refractory to landiolol and verapamil, catheter ablation was performed a few days after the admission, revealing non-reentrant focal atrial tachycardia (AT) originating from the posterolateral region of the right atrium. Homogenization of the origin of the AT was then performed with radiofrequency, resulting in complete suppression of the AT. In the present case, the patient receiving the combination therapy of cetuximab, cisplatin, and 5-FU developed focal atrial tachycardia after chemotherapy, which was successfully treated with the radiofrequency catheter ablation.