This paper proposes a new method for measuring haptic surface roughness remotely. Specially, our target is to measure surface roughness of millimeter order. Proposed method consists of two ultrasound transducers. One is for generating ultrasound. Another one is for detecting sound pressure distribution formed by interfering with reflected waves from a measuring object. A sound pressure distribution changes according to surface roughness of a measurement object. In this paper, we assumed that diffraction occurs on a measuring object surface. Based on the hypothesis, we made a physics model. We conducted fundamental simulations and experiments through the use of the model. We report their results.