In this study, we aimed to achieve a human-adaptive vibration design in a vehicle cabin based on the sensory characteristics of the passengers. To this end, the psychological stress of the passengers as a result of the vibration of the seats in a commercial vehicle was studied. As an initial step, the vibrational acceleration of the seat was measured and its modal analysis was performed. We measured 22 points and 10 points of vibrational acceleration in three directions (vertical, longitudinal, and lateral) of the seat and its occupant, respectively, in a commercial vehicle. We then performed a modal analysis with the obtained data. The results show that the seat had four distinct vibrational modes in the idle state: vertical, horizontal, folding, and rising. The amplitude of the folding mode vibration was 24 Hz, which was consistent with the second-order vibration of the vehicle’s engine. This was the largest vibration of the seat. We also measured the brain and pulse waves of the seat’s occupant to estimate the psychological stress caused by the seat vibration. The results show that psychological stress can be estimated using brain waves. However, pulse waves were not suitable for estimating the psychological stress of the occupant owing to the negative influence of the noise caused by the engine on such waves. We also found that the sensitivity of the occupant to seat vibration can be estimated from the results of spectral analysis of the occupant’s gravity center.