The authors investigate the directivity of sounds radiated by a piano and derive spatial impulse responses for representation of the piano directivity in spatial directions. To obtain the radiation patterns of piano sounds, the radiation field was measured in an anechoic room where a microphone array was placed on a spherical surface with a 2-m radius. The microphones used to measure the radiation field were placed every 10 deg from the top of the piano to 130 deg on the lower side of the piano. The microphones were rounded every 10 deg from 0 deg to 350 deg on the horizontal plane. A reference microphone was placed at the center of the sphere on whose surface the measurement microphone array was placed. The spatial impulse responses were obtained as follows: (1) The power spectra of both the radiated sound in all directions and the reference sound at the reference microphone were calculated; (2) the ratio of the radiated spectrum to the reference spectrum was taken; and (3) minimum phase directivity filters were made using the power spectrum ratios obtained through the above procedures. These directivity impulse responses and filters are useful for binaural room simulations including piano sounds, and for synthesizing piano tones in a computer.