PURPOSE: To obtain an in-line monitor for monitoring the components of a fluid mixture.

CONSTITUTION: The in-line monitor for detecting the presence of one or more component of a fluid mixture and monitoring the concentration thereof comprises at least two substantially monochromatic light sources 2, a probe 10 connected optically to receive a light from the light source and including a light guide path 9 arranged such that the light can act on the fluid mixture, and means 16 for analyzing a light received from the light guide path. One light source functions as a reference light source and the monitor comprises means for dividing the light from each light source to an analyzing part 1b and a control part 18. The analyzing part from each light source is directed to the light guide path while the control part is directed to an analyzing means. Consequently, variation of the entire monitor is compensated for except the absorbance of the fluid mixture.