A device for monitoring a surge arrester, including: a grounding line; a measuring unit including one or more analog measuring circuits for measuring the electric current passing through the grounding line; an electronic processing unit for receiving measuring signals from the measuring circuits; a data storage medium for storing monitoring data produced by the processing unit; a base module, which accommodates the grounding line and the measuring unit; and an auxiliary module, which accommodates the processing unit and the data storage medium and is detachably mountable to the base module. When the auxiliary module is mounted to the base module, the components of the modules are connected to each other through a connector member provided on the base module and a corresponding connector member provided on the auxiliary module.