To provide a data logger and data logger device that can surely guarantee the turn on of a power supply in attaching it to a transported article, make data tampering difficult, and can be reused.

Electronic circuits 11-16 and a switch 21 are stored inside cases 31 and 32. The switch 21 is for turning the power supply on or off to the electronic circuits 11-16, and is turned on or off from the outside of the cases 31 and 32 by combination with a dedicated switch driving piece 5. A holder 40 can attach or detach the data logger 10 and includes the switch driving piece 5. The switch-driving piece 5 turns on the switch 21, when the data logger 10 is mounted.