This study investigated the distribution of floor impact sound levels in the upper and lower floors of an apartment house by the flanking transmission of the sound caused by the impact of a rubber ball. The rubber ball floor impact sound was measured in 10 units (five units on the upper and lower floors based on the excitation floor) of an apartment prior to completion. The measurements indicated that the single number quantity (L''iAFmax) of the rubber ball impact sound on the fifth upper and lower floors based on the excitation floor was reduced by 18dB to 28dB compared to the floor directly below the excitation floor. It was also found that as the sound-receiving floor was further away from the excitation floor, the decrease was greater at the corner sound-receiving point than at the center and at the lower floor than at the upper floor. In addition, it was observed that the lower the frequency band, the greater was the decrease in the floor impact sound level. These results and further investigations would be helpful in resolving conflicts between residents due to misidentification of the location of the floor impact sound in apartments.