Provided is a road surface friction coefficient measuring method and device for measuring the friction coefficient of road surfaces such as roads and runways, and which can be used by being mounted on a general car. This device has a spindle 5 detachably connected to the wheel of a travelling wheel 3 of a car, a support arm 6 oscillatably connected to the spindle, and a measuring wheel supported rotatably by the support arm. Provided to the support arm 6 are a vertical load generating mechanism for applying a vertical load to be added to the self weight of the measuring wheel 8, a first detection unit for detecting the vertical load that is applied to the measuring wheel from the road surface, a rotation transmission mechanism for transmitting to the measuring wheel the rotation of the spindle so as to provide a circumferential velocity difference between the travelling wheel and the measuring wheel, and a second detection unit for detecting the rotational resistance incurred by the measuring wheel from the road surface. The road surface friction coefficient is calculated with a computing unit based on the self weight of the measuring vehicle including the detected values of these detection units and the arm.