The present study was to understand the true power irradiated to the cell line cultured on a culture well, in relation to the nominal power from ultrasonic transducer, and to characterize the temporal variations of the acoustic pressure exerted on the cell. Numerical simulation was carried out for a typical culture well exposed to 1 MHz continuous ultrasound generated by a circular transducer contact underneath the well. The results showed that the ultrasonic pressure exposed to the cell layer in the well was 6.7 times larger than the nominal pressure of the ultrasonic transducer. The ultrasonic pressure in the transient period rose rapidly and was widely variable, and the temporal peak was even greater than that of the steady state period. This suggests that the cells undergo characteristically different ultrasonic exposure between the transient and the steady state period.