To provide a data logger capable of continuously evaluating/analyzing information signals over a long period and to enable data communication by a single logger without causing increase of its size.

The data logger 1 is provided with sensors 11a to 11f to detect the information signals, a microcomputer 10 to process the information signals and a socket 12 to alternately connect an attachable/detachable medium 3 and a communication card 4 to the microcomputer 10 and constituted so that the information signals are recorded in the medium 3 at a reprocessible state in the case of connection of the medium 3 and the information signals are transmitted to other devices in the case of connection of the communication card 4.