We conducted observations of wind velocity profiles above a high-density area in Tokyo, Japan, using a Doppler LIDAR system. The observation data of an exponent index for the power law, which is commonly used to describe the wind velocity profile, displayed diurnal variation, decreasing in the daytime. Building simulations considering diurnal variation of the exponent index are not often performed, and most cases use a constant value. This paper provides information on the error in the calculated ventilation airflow rate due to the use of a constant value for the exponent index, on the premise that a variation of an exponent index obtained from observation is the true value. The error in the calculated ventilation airflow rate was quantified based on comparison of the ventilation airflow rate calculated using a constant value of 0.22, and the ventilation airflow rate calculated considering a diurnal change in the exponent index. The results indicate that the ventilation airflow rate obtained from a constant value for the exponent index for an isolated building with two openings is underestimated by up to 8% in the daytime and overestimated by up to 14% in the nighttime. (C) 2016 Elsevier B.V. All rights reserved.