In Japan, there are many houses built in limited space as well as in close proximity to causeways and railroads, due mainly to concentration of population in the cities in recent years. Slender structures and especially 3-story houses are susceptible to vibration induced by external forces such as traffic. This effect often creates an uncomfortable environment for habitation. Because houses often have a complex geometry, it is difficult to establish the vibration characteristics of a structure. Furthermore, the response characteristics of a house will change with environmental conditions. A prototype Active Mass Damper (AMD) has been developed to address some of these issues. This has been tested in an exhibition house in Tokyo. It uses two AMD’s of 1,725N mass each for both X-axis and Y-axis. Both AMD’s are applied in X and Y directions and are installed at RooF level, they are controlled by Direct Displacement Feedback (DDFB). The test showed that the AMD could dramatically reduce vibration. Of particular note was reduction about 6dB to 8dB (L10) in the first mode of vibration even though the house mass is double the prototype target weight. Also, vibration was dramatically decreased by the AMD on 3rd floor of the house. Vibration level and comfort are important criteria for this kind of system.