Effects of nonlinear properties of surface soil on the variation in strong ground motion and damage to wooden houses within Ojiya city during the 2004 Mid Niigata Prefecture earthquake, are examined, based on field investigation including boring and microtremor measurements, as well as strong motion observation. It is revealed that the maximum response of wooden houses, i.e., damage level, could mainly be controlled by strong ground motion with a period of about 1-2 s. Such a period range was significantly amplified in the damaged sites due to strong soil nonlinearity while it was not in the slightly damaged sites due to week soil nonlinearity.