PURPOSE: To rapidly cope with even a change in a rapid change in ventilation environment owing to a disturbance element and to produce a ventilation comfortableness by providing a computing device which computes an operation amount through fuzzy logic computation based on state information by a detecting means and a control rule fetched from a memory device.

CONSTITUTION: By means of state information of ventilation environment grasped by a detecting means 1 and a control rule based on an experience rule fetched from a memory device 3, an amount of operation made on the electric motor for blast of a ventilating fan is decided by a fuzzy inference processor 4 through fuzzy logic computation. Ventilating operation is performed by means of the decided operation amount, coping with rapid change in ventilation environment owing to a disturbance element is effected, and a comfortable ventilating is produced.