[Objective]Our simulation laboratory holds four JRC-certified ICLS classes per year. In these classes we examined the depth and quality of chest compressions. [Method] Ten residents, 4 clinical laboratory technicians, 15 nurses, and 1 doctor participated were studied. We measured the average depth of chest compressions and the ratio of compressions less than 5 cm on the ALS Simulator Ambu®man 2016 Advance (Ambu, Dnmark) for VF/PLVT, PEA/cardiac arrest, and Megacode scenarios (cardiac arrest 4 waveform composite cases). [Results] Average chest compression depth was 5.53±0.54 cm for VF/PLVT, 4.94±0.81 cm for PEA/asystole. The percentage of compressions less than 5 cm was 24.6% for VF/PLVT, 47.0% for PEA/asystole. We observed a significant difference in the mean depth of VF/PLVT:PEA/asystole (p = 0.0297), and for the ratio of compressions less than 5 cm VF/PLVT:PEA/asystole (p = 0.0302). [Discussion] In all groups, in cases of PEA/asystole, average chest compression depth was shallower and the proportion of compressions less than 5 cm increased, due to the differential diagnosis. Therefore, when the cause is not clear, CPR needs to be performed more carefully so that the depth of compressions does not become too shallow.