Of the 61 animals diagnosed with colibacillosis on a broiler farm, 40 were treated with the oral administration of enrofloxacin (ERFX) over 3 days, and the remaining 21 were not treated. The number of dead and culled broiler chicks in each group was recorded for 9 days before and after the administration of ERFX. The number of dead and culled chicks was converted to the assumption that the number of chicks reared on the day of diagnosis was 10,000, and then the efficacy of ERFX was evaluated. As a result, in the early age group (7 to 28 days old), a decrease in the number of dead and culled chicks was observed from the 2nd day after the start of administration. Also, in the group closer to the late age group (33 to 38 days old), a decrease was observed from the 2nd day after the start of administration, but the dispersion was smaller than for the late age group. However, in the non-treated case group (27 to 38 days old), the number of dead and culled chicks continued to increase after diagnosis, and then showed a tendency to slowly decrease.