security firm; security company
A company that provides security services, such as guarding buildings, monitoring alarms, or transporting valuables.
That security firm is in charge of security for this building.
警備会社 is a more common and shorter term for a security company. 警備保障会社 is a longer, more formal variant that explicitly includes 保障 (guarantee/protection).
Compound of 警備 (security), 保障 (guarantee/protection), and 会社 (company).