To provide a data logger that can make a highly reliable calculation of the time during which an object is placed in an improper environment.

A data logger comprises: threshold comparison means that compares a measurement value obtained by measuring an object to be measured at a predetermined timing with a threshold and outputs comparison data as the comparison result; previous comparison data storage means that stores previous comparison data being comparison data obtained in the measurement immediately previous to the ongoing measurement of the measurement value; control means that generates an execution request for calculating an improper time according to the combination of the comparison data and the previous comparison data and updates the storage contents of the previous comparison data storage means by setting the comparison data as the previous comparison data; and improper time calculation means that calculates, according to the execution request, the sum of improper times that are the time during which the object to be measured was placed out of a proper environment.