A device is provided for ascertaining the internal delay time of a CAN bus connection unit in a bus system including user stations exchanging messages via a bus, the transmission access to the bus for each message being assigned using an arbitration method according to CAN standard ISO 11898-1 to one user station which becomes the transmitter for this message. The device includes a unit for ascertaining the internal delay time using one of (i) a delay counter for ascertaining the delay time between a transmission signal and a reception signal, the delay counter being stopped if the reception signal has a dominant level and the counter content of the delay counter is greater than/equal to a predefined configuration value, or (ii) the maximum and minimum delay times of the bus connection unit.