A 71-year-old man was admitted to our hospital with a hard, irreducible, and enlarged right inguinal mass with a diameter of 1-cm. Ultrasonography (US) revealed a saccular structure 2.5-cm in diameter that stemmed from the abdominal cavity containing a tubular organ with a blind tip, net-like area, and anechoic area ; these features were identified as the appendix, greater omentum, and ascites, respectively. US also revealed a wall deficit in the appendix, while color Doppler imaging showed an absence of blood flow at the distal appendix. Perforated appendicitis was suspected, and surgery was performed 25-hours after clinical presentation. Surgery revealed an appendiceal perforation and abscess in the hernial sac, which were treated via appendectomy followed by McVay repair. Our case findings reveal that US is a useful tool for diagnosing De Garengeot hernia and identifying perforated appendicitis.