To solve a problem that when attaching a conventional security closure device, a removal of an established mail slot is required and that is troublesome.

This security closure device includes a shell 1 formed by improving a three-dimensional shape having a C-shaped cross-section into a specific shape, a shell lid 2 formed by improving a U-shaped flat shape into a specific shape, and a connecting means for connecting both to each other. The security closure device can encase an established mail slot 11 into a compact shape and is easily fixed to a door front surface with existing screws 13.