1. Efficiency is defined as the ratio of the amount of output to that of input and psychological efficiency is considered to have two components, that is 1.) objective efficiency in which time actually required, the amount of investment etc. are the index of input and the amount and the utility of the products etc. are the index of output, and 2.) subjective efficiency in which the performer's psychological time, and his feeling of efforts, satiation and fatigue etc. are the index of input and the performer's feeling of satisfaction in the products and his own behavior etc. are the index of output. 2. An experiment is reported in which among various ways of doing simple task differences are found in subjective efficiency but not in objective efficiency. 3. A general principle concerning human behavior in task situation is suggested. That is : in a situation in which he has to do a task man behaves so as to maximize psychological efficiency.