Two-dimensional motion of a piano string has been observed. When the hammer moves to strike the string, the irregularity of the surface causes horizontal as well as vertical vibration of the string. It has been thought that the vertical and the horizontal vibration dominates, respectively, the prompt and after sound of a piano [G. Weinreich, Sci. Am. 240, 118--127 (1979)]. An apparatus was made to observe the two-dimensional motion, assembling two optical fibers and two phototransistors. The wrapped single string E1 was measured with this equipment. The following results were found. (1) The amplitudes of the horizontal and vertical vibrations repeat, increasing and decreasing during the after sound. This means the sound is formed by the coupling of both vibrations, which occurs at the inclined pins of the bridge. (2) When the string is struck by the hammer, it vibrates at first vertically to the soundboard and then, after few seconds, it starts to rotate. The direction of the rotation changes alternately about every 6 s. (3) The first direction of the rotation is not determined. It depends on how one touches the key. This means the touch may change the piano timbre.