In this research, "Data Assimilation" was applied to a wind tunnel experiment. A square cylinder wind tunnel experiment and the ensemble Kalman filter were used to integrate surface pressures of a square cylinder obtained from computation and experiment. The results indicated that surface pressures of the square cylinder and flow field can be reproduced by the present data assimilation method, while they could not be replicated by numerical simulation alone because of the coarse grid resolution. The results of the ensemble Kalman filter were compared with those of "Measurement-Integrated Simulation," which is another integrated method between computation and experiment for fluids. From the comparison results, it was considered that the ensemble Kalman filter has advantages in the region around the wind tunnel model compared to the measurement-integrated simulation.