A production line monitoring device that identifies a cause of a production defect, reduces the amount of analysis data and computation, and performs real-time processing, is provided. The production line monitoring device includes a defect indication detection unit that detects an indication of a production defect of a production line, and a defect cause identification unit that identifies a cause of the production defect. The defect indication detection unit collects measurement information measured by an inspection apparatus for each reference that identifies a position on products, and detects an indication of the production defect from the change with time of the measurement information at the references. The defect cause identification unit performs stratified analysis based on production information related to the reference when the defect indication detection unit detects an indication of a production defect, and identifies a cause of a production defect from a result of the analysis.