Provided is a highly clean environmental system capable of obtaining a clean space with an extremely high cleanliness of class 1 or higher by an extremely simple configuration without using a huge clean room, maintaining the clean space at the same oxygen concentration as that in an installation environment, and implementing the application thereof to human medical treatment, medical examination, recuperation, and the like easily and at low cost. In this highly clean environmental system, a closed space that can be hermetically sealed by an enclosure the interior of which can maintain a clean environment is formed. A fan filter unit is disposed in the closed space. A suction port and/or a discharge port of the fan filter unit is configured so that the relative position thereof with respect to the closed space can be changed. The larger of either the maximum value of the distance between the suction port and the discharge port or the size of the discharge port of the fan filter unit is defined as x. When the length of the closed space in the direction in which x is defined is defined as X, there is at least one direction in which the ratio x/X is 0.3 or more.
