A method of evaluating the gas viscous friction force acting on head/disk interface has been developed. In the past, the effect of the gas viscous friction force has been almost negligible, due to its small value compared with the contact friction force. Recently the gas viscous friction force has tended to increase with the decrease in spacing and the increase in relative speed between the slider and the disk, therefore it is becoming necessary to consider its effect on motor load or slider posture. Few experimental studies of the gas viscous friction force, however, have been performed. In this study, the measurement of the gas viscous friction force has been realized by developing a sensitive friction force sensor. Furthermore a method of evaluating the gas viscous and contact friction forces separately has been also established.