Biological studies of human consciousness based on recent neuroimaging experiments, i.e., functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), were reviewed from the viewpoint of different functions of consciousness. A biological hierarchy of consciousness structured in three levels, i.e., arousal, awareness and self-consciousness, was reviewed in connection with working memory. We found that the dorsolateral prefrontal cortex (PFC), anterior cingulate cortex, medial PFC, and parieto-temporal junction area play a crucial role in social cognition involving estimation of the mental states of other people. The ventrolateral PFC plays a role in language-based rehearsal/imitation in connection with a mirror system. Frontal pole and orbitofrontal areas are also likely to contribute to generation of self function, reward expectancy and internal planning of goal-directed behavior. Interestingly, we found that these various PFC and related areas strongly contribute to active consciousness based on the working memory system. Furthermore, we have shown that a theory-of-mind approach could be closely related to higher cognitive functions involved in working memory, which has a meta-recognition processes during mentalization.