The invention relates to a device for monitoring a surge arrester connected to a power network, comprising a registering unit (1) and means (63, 66) for attachment of the registering unit to the surge arrester. The registering unit comprises a grounding line (16), which is adapted to conduct a current flowing through the surge arrester to ground, means (4) for registration of a current passing through the gounding line (16) from the surge arrester, a member (9) for registration of current pulses passing through the surge arrester, and a storage member (5) for storing data concerning the current passing through the grounding line (16) from the surge arrester and data concerning registered current pulses. The registering unit (1) further comprises a field probe (17) connected to ground, a sensor (18) for registration of the current flowing between the field probe (17) and ground, and a time meter (3), the storage member (5) being adapted to store the data concerning the current passing through the grounding line (16) from the surge arrester and the data concerning the current flowing between the field probe 817) and ground associated to a time information determined by means of the time meter (3). The invention also relates to a system comprising one or several such registering units.