Normal and tangential fluctuations in sliding friction are examined by using a newly-designed tester, the tribovibrometer. A periodically grooved specimen is used in order to emphasize the information of the surface shape in the signals of sliding friction. The fluctuations in sliding friction include two kinds of components, i.e. the components of forced vibration and self-excited vibration. At least, the former includes the information of the surface shape of mating surfaces. The amplitudes of tangential fluctuations are much larger than those of normal fluctuations. The ratios of the amplitudes of forces in tangential and normal directions of AC components are above unity, which is much larger than those of DC components, i.e. mean friction coefficient.