PROBLEM TO BE SOLVED: To always determine a dehydrated state without imposing a load on a person to be measured.SOLUTION: A dehydrated state determination device has blood flow measurement means for receiving return light of laser light radiated on a wrist of a person to be measured, and analyzing the return light to measure a blood flow. When the person to be measured lifts a hand at a certain vertical interval h and stands still, the dehydrated state determination device measures a restoration blood flow BFr and a blood flow restoration time Tr from reversal to restoration according as the blood flow falls once, reverses to rise, and is restored to be constant, and divides the restoration blood flow BFr by the blood flow restoration time Tr to calculate a blood flow restoration gradient H. The dehydrated state determination device determines whether or not it is a dehydrated state according to whether or not a water ratio reduction degree Rm obtained by dividing a reference blood flow restoration gradient Hs calculated according to auxiliary measurement in normal time of the person to be measured by a blood flow restoration gradient Hp calculated according to measurement in ordinary time of the person to be measured is a prescribed reduction degree threshold value or greater, and when it is determined that it is the dehydrated state, performs voice output of a water supply message to prompt a water intake.
