The mechanism of a passive solar heating system and its relation to the mechanism of the global environmental system are discussed using the concepts of entropy and exergy. The reason that we use both of these concepts is to make it clearer how the systems work and hence why the passive solar heating system is called environmentally friendly. The mechanism of the systems is described as a process in which exergy is supplied, its portion consumed, and the resultant generated entropy is finally given off. We call this process ＇＇exergy-entropy process＇＇. A numerical example of the exergy-entropy process of the passive solar heating system and the global environmental system revealed the following. Generated entropy within the passive solar heated room is discarded into the outdoor environment through glass windows and concrete floors. The global environmental system has a mechanism that disposes of all the generated entropy resulting from the solar exergy consumption on the Earth＇s surface into the universe. Passive solar heating systems therefore fit inside the global environmental system, on which they impose almost no burden.