Pneumoperitoneum that develops in the absence of a perforated abdominal viscus and does not require laparotomy is considered to be idiopathic. Differentiating between idiopathic pneumoperitoneum and highly lethal perforation due to necrotizing enterocolitis in preterm infants is important. Herein, we report two cases of idiopathic pneumoperitoneum in preterm infants who underwent exploratory laparotomy and conservative treatment, respectively. The first patient was born at 32+5 weeks of gestation and developed pneumoperitoneum on day 7 of life. The patient underwent exploratory laparotomy and was diagnosed with idiopathic pneumoperitoneum after surgery. The second patient was born at 30 weeks of gestation. He developed pneumoperitoneum on the eighth day of life. Idiopathic pneumoperitoneum was suspected, and the patient was treated conservatively without laparotomy. Based on our awareness and experience of the first case of idiopathic pneumoperitoneum, we were able to treat the second patient conservatively. These cases will be helpful for diagnosing and treating pneumoperitoneum in preterm infants.