Realized is a solid substance concentration managing method which allows quick detection of an abnormality in a chemical reactor. The present invention is an invention of a solid substance concentration managing method of managing a concentration of a solid substance which is contained in a residue that is discharged in a reaction product gas processing step included in a trichlorosilane producing method, the solid substance concentration managing method including a concentration measuring step of measuring the concentration of the solid substance which is contained in an after-crystallization residue that is obtained by crystallizing part of aluminum chloride.