Accelerometers are recognized as a valid and objective tool for assessing free-living physical activity and have been used extensively to monitor activity patterns in field settings. When processing accelerometer data, several issues should be considered, such as identifying daily wear time. This study examined the differences in the number of valid data gathered with different decision rules. The participants were 213 overweight adults who were recruited for a weight-loss intervention study. For a baseline assessment of physical activity, they wore a triaxial accelerometer HJA-350IT for 14 consecutive days. A valid day was defined as having 10 h of wear time. Wear time was estimated by subtracting non-wear time from 24 h. The accelerometer collected acceleration signals over a 10-s time interval. First, non-wear time was defined as the time during which there was no acceleration signal. The number of participants who had valid records for more than 2 weekdays and 1 weekend day was 204. Next, non-wear time was defined as the time during which there was no acceleration signal continuously for 20 min or more. In this case, the number of participants who had valid records under the same condition was 210. These findings suggest that the choice of wear time makes a difference in the number of valid data.