PURPOSE: To facilitate control of water supply of water into a water container by using a very compact means to detect the water level in the water container.

CONSTITUTION: In a humidifier comprising a number of hollow pipes 11, 11... each made of a porous material permeable to steam but impermeable to water and disposed parallel to one another within a water container 12 with air passages formed within the hollow pipes 11, 11... and a water supply means 13 for supplying water into the water container 12, the water container 12 is transparent. Optical sensors 16, 17 for detecting the water level in the water container 12 are disposed at the upper end portion of the water container 12 and a control means 18 is provided for controlling supply of water by the water supply means 13 according to signals from the optical sensors 16, 17.