To provide a mental stress judging device capable of reducing errors based on the sudden change of a heart rate.

This device is provided with a means for obtaining the heartbeat equivalent signal and the respiratory vibration equivalent signal of an examinee, a first conversion means for performing time/frequency conversion to time-axial heartbeat interval data of the heartbeat equivalent signal, a data specifying means for specifying only data in a frequency band equal to or lower than a prescribed frequency among the pieces of data converted by this first conversion means, a second conversion means for converting frequency/time to data specified by this data specifying means, a movement averaging means for performing movement averaging processing to the time-axial heartbeat interval data converted by this second conversion means based on the respiratory vibration equivalent signal, and a stress analytic means for analyzing stress based on the heartbeat interval data movement-averaged by this movement averaging means.