A mental stress detection device (10) includes an index value calculation unit (200) and a correlation calculation unit (300). The index value calculation unit (200) calculates standard deviation (SDn) of heartbeat intervals (RRIn), a root mean square (RMn) of a difference RDn between temporally-adjacent heartbeat intervals (RRIn), and a ratio (SDn/RMn) between the standard deviation (SDn) and the root mean square (RMn). The root mean square (RMn) correlates with an activity of parasympathetic nerves and the ratio (SDn/RMn) correlates with an activity of sympathetic nerves. The correlation calculation unit (300) calculates a moment correlation coefficient (rn) which is a correlation between the root mean square (RMn) and (the ratio SDn/RMn) and a correlation associated with time.
