Because many veteran operators retired in chemical industries, it is very important to educate and train efficiently novice operators to have the basic operation skills. The authors conducted two kinds of experiments using a semi-scale thermal power plant emulator to investigate the factors to induce violations and to evaluate the applicability of a training manual with a checklist of operation steps. The results related with operation errors including the violations of operation rules such as finger and calling, sitting on a chair at maneuvering on the operation panel, etc. showed different tendency between the experiments. This paper compares the numbers and types of operation errors in the two operation experiments to investigate the causes of the difference. The comparison results suggest that pointing out operation errors after the completion of an operation task has positive effect for acquiring basic operation skills.