Pneumoperitoneum often occurs after the perforation of the gastrointestinal tract. However, pneumoperitoneum without the perforation has been reported as one of the complications of collagen diseases, the cause of which is usually the rupture of pneumatosis cystoides intestinalis (PCI). PCI is sometimes observed in the patients with scleroderma and mixed connective tissue disease but rarely in the patients with systemic lupus erythematosus. We reported here a case of systemic lupus erythematosus developed the pneumoperitoneum without the perforation of gastrointestinal tract. A 51-year-old female who had been diagnosed as systemic lupus erythematosus and taken steroid for 12 years, visited our hospital because of general malaise. She had no abdominal symptoms but the roentgenographic examinations revealed the pneumoperitoneum. The laparotomy was performed and there were no findings of the perforation of the gastrointestinal tract. Because PCI is hardly recognized macroscopically after the rupture and the pneumoperitoneum due to PCI is often asymptomatic, we considered the cause of the pneumoperitoneum in this case was the rupture of PCI. The mechanisms of the formation of PCI in patients with collagen diseases were also discussed in this paper.