In order to search trends in further energy saving in highly insulated and airtight houses in Hokkaido, Japan, an actual-condition investigation on a central-heating system fitted in five such houses, each with different methods of construction, was performed. The results were obtained as follows.:1)The equivalent leakage area value of a steel frame construction was 3.2cm2/m2 that was less than a two-by-four construction(0.33cm2/m2) and a conventional wood construction(1.1-1.7cm2/m2) .It is important to be improved air-tightness of steel frame construction houses by various construction methods. 2) Heat loss coefficient estimated from these experimental results was 0.94 -1.59W/(m2•K). To reduce energy consumption for heating,it will become more important to improve thermal-insulation properties and air tightness.3) The heating-system efficiency obtained from these experimental results was in the range of 72 to 93%. Although the ratio of actual boiler capacity to demanded heat-source capacity extrapolated from actual measurements ranges from two to three times,the effect on system efficiency was less than 10%.However it is necessary to include this 10% in the task of planning a heating system.