PURPOSE: To display the concentration of noxious material by only one light source, by controlling light which passes through a plurality of optical conductors by optical switches in response to the measured concentration signal of the noxious material.

CONSTITUTION: The optical switches 8-1W8-n are selectively controlled in response to the measured concentration signal of the noxious gas obtained by a gas concentration meter 10. The light emitted from the light source 5 of a light generating device 4 is inputted into one end of each of the optical conductors 3-1W3-n and reaches the other ends. At the other ends of the optical conductors 3-1W3-n provided at the position of a front plate 1, only the end surfaces selected by the optical switches are lighted, and the concentration of the noxious material is digitally displayed by dots.