Disclosed are method and apparatus for measuring concentration, that can simultaneously measure gaseous material concentrations and solid particulate material concentrations and further can simultaneously measure the concentrations of a plurality of materials such as black smoke, white smoke, and water vapor in the solid particulate material concentrations in a simple and reliable manner. The method for measuring concentration comprises applying a laser beam having an absorption wavelength inherent in a gaseous material contained in an object to be measured, to the object to detect a light transmittance and a light absorption amount and detecting the concentration of gaseous materials in the object and the concentration of solid particulate materials in the object. The relationship between the concentrations of a plurality of kinds of solid particulate materials including black smoke and white smoke and a laser beam attenuation level in each absorption wavelength is preset. Laser beams having a plurality of respective absorption wavelengths are applied to the gaseous material to be measured. The attenuation levels of the applied laser beams having the plurality of absorption wavelengths are measured. The measured attenuation levels are compared with the attenuation levels calculated based on the present relationship to calculate the concentrations of the plurality of kinds of solid particulate materials.