Most research on the relationship between driver fatigue, driving comfort, stability and other factors related to driving has consisted subjective assessments via surveys. Bio-signal measurements allow more objective evaluations but the measurement processes and types are very complicated, and measurement signals are sensitive to noise. To address this issue, this paper attempts to quantify driver fatigue by measuring and analyzing hemodynamic responses with near-infrared spectroscopy. The oxygen concentration changes in the erector spinae muscle were measured for the same subjects when driving a vehicle on a highway for a long time, in one of two driving postures: posture A based on the seat status of the average male body type in South Korea (a 107.7° torso angle, 109.1° chair back angle and 561.0 mm seat x-axis position posture) and Posture B with the chair back angle adjusted by −10° from posture A and the seat moved 4 cm back. A survey was carried out on the degree of lumbar discomfort. The analysis of the degree of relative fatigue, based on driver posture utilizing a mathematical model for the physiological system of lower-back fatigue, showed that posture B is relatively uncomfortable compared to posture A by approximately 41%. The results of analyzing the degree of relative discomfort with regard to driver posture using a mathematical model allow the verification that interpreting the relative degree of discomfort based on the changes in the oxygen concentration at a particular time is possible, and the experimental results show the usefulness of the proposed method.