A highly reliable vehicle driver's fatigue evaluating method is provided, which is capable of quantitatively calculating a degree of fatigue of a driver seated on a seat. The degree of fatigue of the driver seated on a seat is quantitatively calculated based on an amount of rearward deflection of a lower part of a backrest portion of the seat, a load applied downward to a front part of a seating portion of the seat, and a load applied rearward to an upper part of the backrest portion, in a state of the driver being seated on the seat.