1. A failure determination system for determining failures of a plurality of temperature sensors provided in an internal combustion engine, comprising: stoppage detecting means for detecting whether or not the engine has stopped; initial temperature-storing means for storing respective temperatures detected by the temperature sensors when stoppage of the engine is detected, as initial temperatures; dimensionless value-calculating means for making dimensionless the respective temperatures detected by the temperature sensors after stoppage of the engine based on the corresponding initial temperatures, to thereby calculate respective dimensionless values of the temperatures detected by the temperature sensors; estimated normal value-setting means for setting in advance an estimated value of a dimensionless value of a temperature to be detected when each of the temperature sensors is normal, as an estimated normal value; and failure determination means for determining failures of the temperature sensors based on results of comparisons of the respective calculated dimensionless values of the temperatures detected by the temperature sensors with the set estimated normal value.