A concentration presence/absence determining device includes: a brain blood flow volume obtaining unit configured to obtain a brain blood flow volume of the user； a varying threshold obtaining unit configured to obtain a varying threshold that is a value smaller than the brain blood flow volume of the user at a base time； and an determining unit configured to determine that the user concentrates, when the brain blood flow volume obtained by the brain blood flow volume obtaining unit falls below the varying threshold obtained by the varying threshold obtaining unit.
