To provide a low cost double door system that can effectively prevent a prowler's intrusion.

The double door system comprises a first door providing an external access to a building, peripheral walls cooperating with sidewalls of the building in enclosing the first door to form a closed space outside the first door, a second door installed in the peripheral walls to provide an external access to the closed space and enable the closed space to be viewed from the outside, and a mirror disposed in the closed space to reflect a human in front of the first door.