Abstract: A friction test apparatus with rotating friction means and endless belt set around one pair of rotating drums, has a meandering prevention mechanism to prevent the belt from meandering; and a belt supporting means which has a flat stage facing the inner face of the belt and a water-supplying mechanism which supplies water between the flat stage and the belt from an upstream side of belt rotation direction to the flat stage. Said apparatus also has a sample holder driving means which has a three-component detecting portion which detects loads on the sample in front-rear direction, left-right direction and upper-lower direction, and the sample holder driving means is having a slip angle regulation mechanism for regulation of a slip angle of the sample attached to the sample holder to the belt, and constructed so as to adjust the slip ratio of the sample attached to the sample holder to the belt.