An anti-focus is defined as the point where the pressure amplitude is zero. As an active protection means in focused ultrasound, the anti-focus was researched through simulations. A 513 element, 1 MHz, 15 cm focal depth, spherical shape transducer with a central hole was used for the simulation. The phase and amplitude condition to create pressure field was calculated by pseudo inverse method with the control value of 1 for a focus and 0 for an anti-focus. The lateral direction minimum distance between a focus and an anti-focus was determined by the comparison between the intended control values and the simulation results. Additionally, pressure fields of four foci symmetrically located at 3 mm from a central anti-focus and the same four foci without the anti-focus were simulated. With the calculated fields, temperature simulations were conducted based on bio-heat transfer equation. Damage was estimated from the calculated temperature profile with the thermal dosage of 240 equivalent minutes at 43 °C. The field simulation results indicate that the minimum distance is approximately 3 mm. The distance is approximately the end of the first side lobe of the transducer. The temperature simulation results show that the entire area of approximately 36 mm² was estimated as the damage zone after 280 millisecond sonication of four 4 MPa foci. However, approximately 2 mm radius central zone was saved from the thermal damage by the addition of an anti-focus. Anti-foci are expected greatly to reduce the collateral damage in focused ultrasound through improved field control.