Provided is a production line monitoring device which is capable of identifying, with high accuracy, a cause of a production defect, and which reduces the amount of analysis data and the amount of computation therefor and allows real-time processing. This production line monitoring device (6) is provided with a defect indication detection unit (61) which detects an indication of a production defect in a production line (1), and a defect cause identification unit (62) which identifies a cause of the production defect. The defect indication detection unit (61), for each of references (REF1 to REF3) which identify positions in a product, collects measurement information measured by a measurement device (5), and detects an indication of a production defect on the basis of variation over time of measurement information in the references (REF1 to REF3). The defect cause identification unit (62) performs stratified analysis on the basis of production information related to the reference (REF2) when the defect indication detection unit (61) has detected an indication of a production defect, and identifies the cause of the production defect on the basis of the analysis result.