Introduction: Every year, about 600 million people in the world get infected foodborne diseases and 420,000 people die each year due to these diseases. Methods: This study was conducted to investigate a gastroenteritis outbreak in one military unit located in the northeast of Iran in May 2017. In order to determine the source of the disease, a line listing was prepared and completed by 296 staff of the unit. Then, the source of the outbreak was identified by calculating the relative risk for the meals consumed within 48 hours before the beginning of the outbreak and using the stratification method to adjust the confounding effect. Results: In this outbreak 323 people were infected. The disease attack rate was 22.4%. 51% of staff were infected. The most common symptoms of the disease were fever, diarrhea and headache, respectively. Only the relative risk of boiled eggs was significant and was identified as the source of the outbreak (RR = 2.4). The cause of the outbreak was Shigella and less likely Salmonella. Conclusion: The long-time gap between the cooking of dinner and its distribution has been one of the underlying factors of the outbreak. It seems that the boiled eggs remain in the lukewarm water inside the pot for several hours before being distributed to the staff and the secondary contamination of the pot containing the eggs was the cause of this outbreak.