A piston temperature state monitoring system for an internal combustion engine (M10) which monitors a frequency at which a temperature of a piston of the internal combustion engine becomes equal to or higher than a monitoring setting temperature set in advance or calculated, comprising: a piston temperature calculation system (M11); a piston temperature suppression control system (M12); and a count output system (M13), wherein the piston temperature calculation system (M11) measures or estimates a piston temperature, the piston temperature suppression control system (M12) receives the piston temperature and, when the piston temperature is equal to or higher than a control start temperature (Tc), the piston temperature suppression control system (M12) performs a piston temperature suppression control where fuel injection into a cylinder is controlled to suppress an increase in the piston temperature based on a maximum limit temperature, in which the piston temperature is set in advance, and the control start temperature (Tc), which is the value lower than the maximum limit temperature and set in advance or calculated every moment, and if a ratio of a temperature difference (ΔT) which is the difference between the maximum limit temperature and the piston temperature to a margin width (ΔTm) which is the difference between the maximum limit temperature and the control start temperature (Tc) is equal to or less than a count threshold ratio (μc) set in advance or calculated, the count output system (M13) outputs the count signal (Cs) so that the number of times of the piston temperature suppression control is counted; wherein the count threshold ratio (μc) is set such that the count signal (Cs) can be transmitted at a timing when the piston temperature suppression control is reliably performed and the count threshold ratio (μc) is set to a value that is greater than 0.0 and less than 1.0. a piston temperature calculation system (M11); a piston temperature suppression control system (M12); and a count output system (M13), wherein the piston temperature calculation system (M11) measures or estimates a piston temperature, the piston temperature suppression control system (M12) receives the piston temperature and, when the piston temperature is equal to or higher than a control start temperature (Tc), the piston temperature suppression control system (M12) performs a piston temperature suppression control where fuel injection into a cylinder is controlled to suppress an increase in the piston temperature based on a maximum limit temperature, in which the piston temperature is set in advance, and the control start temperature (Tc), which is the value lower than the maximum limit temperature and set in advance or calculated every moment, and if a ratio of a temperature difference (ΔT) which is the difference between the maximum limit temperature and the piston temperature to a margin width (ΔTm) which is the difference between the maximum limit temperature and the control start temperature (Tc) is equal to or less than a count threshold ratio (μc) set in advance or calculated, the count output system (M13) outputs the count signal (Cs) so that the number of times of the piston temperature suppression control is counted; wherein the count threshold ratio (μc) is set such that the count signal (Cs) can be transmitted at a timing when the piston temperature suppression control is reliably performed and the count threshold ratio (μc) is set to a value that is greater than 0.0 and less than 1.0. <EOS>