A previously healthy 6-year-old boy had continuous fever for 6 days before admission to our hospital. His general condition was good except for pyrexia. The left lobe of the thyroid gland was swollen, red, hot, and tender, and neck movement was limited. The provisional diagnosis was upper respiratory tract infection. We demonstrated the existence of an orifice of the left piriform sinus by esophagography in this case, and made a diagnosis of acute suppurative thyroiditis caused by a piriform sinus fistula in the hypopharynx. The causative organisms of acute suppurative thyroiditis include Peptostreptococcus, Staphylococcus haemolyticus, and α-streptococcus, but the organisms responsible are commonly undetectable in clinical cases. We detected Eikenella corrodens in the present patient. Although Eikenella species occasionally causes acute suppurative thyroiditis in adults, it is rare for this to happen in pediatric patients. Antibiotics were administered for 7 days. We also performed aspiration of the abscess on the 8th day of the illness. The abscess was reduced in size and tenderness was relieved after aspiration. In conclusion, if a pediatric patient has swelling, heat, tenderness, and redness of the anterior neck, we should keep in mind acute suppurative thyroiditis and the possibility of a fistula. If there is an abscess, we should immediately perform aspiration, culture, and isolation, and choose the appropriate antibiotics for the causative bacteria.