The seriousness and importance of human errors as the cause of the accidents is widely recognized in the almost all industries. However, the establishment of the errors prevention measures, which lead to the accident prevention directly, effectively and concretely, seems to be difficult. Because, the causes and the background factors of human errors are complex and diverse. Although it is difficult to get these factors clear, they must be managed appropriately and properly to establish the appropriate measures for the accidents prevention. Human errors are not unusual characters to be inherent in the specific individuals or environments, and they are certain results influenced by each occasion and situation. Strictly, the characteristics of human reliability, accuracy, vitality, and so on, always changes, so our any acts might to become errors depending on the circumstances. To attempt the accident prevention measures caused by human errors, it is necessary to grasp the process of the accident-occurrence in detail. Especially, what relation the human factors had had among the various accidents causes, and how the psychological condition of victim had been, should be considered carefully. In this research, four labor accident-cases are extracted from 191 accident-cases which occurred in 2000 in Japanese construction work. And the relations among the psychological factors of victims, the processes of the accident-occurrence, and error factors are discussed, including the supposition of the researcher. For example, some simple questions would arise from these labor accident-cases as below: • Usually, how many risks are informed to us by a warning? • How much space is necessary for you to close the gate of a dump truck? • Would you approach the window side when a cellular phone is not connected? • To prevent the falling, is it an error to bind luggage to the frame of the lift with the rope? Even in these four accident-cases which were extracted in this research, each error was simple, and each psychological factor was common to everyone. And in each accident-case, some factors would be connected with each other like a chain. In other words, the accident would be hard to occur by the single factor. The prevention of the accident by human errors might be difficult, and the accident might occur as long as the chain of these factors could not be severed. Therefore, in order to prevent the accidents, it is necessary to understand human errors not as the numerical values, but as a series of phenomena resulted by human behaviors.