For supervising the security of a computer system (SY) comprising several elementary computer items (BI), such as machines and applications, and several gathering items (BIg), such as networks, services or sites, gathering elementary items, a supervision device (DS) collects base measurements (MB) representative of states of the elementary items. A unit (UDI) determines several security indicators (I) of different types for each elementary item according to respective functions of the base measurements and several security indicators of different types for each gathering item. Each security indicator of a given type of a gathering item is determined according to a respective function of the security indicators of the given type of the elementary items gathered in the gathering item. The indicators of one item relate to the availability, the intrusion, the vulnerability and the compliance to a security policy.