The present invention provides a method comprising allowing a reaction of a sample, a reagent containing Factor C, which can be activated by binding with endotoxin, and a synthetic luminescent substrate comprising a luminescent substrate bound to a peptide, for release of the luminescent substrate from the synthetic luminescent substrate, allowing a luminescent enzyme to act on the luminescent substrate released in the luminescent substrate release step, for measurement of the luminescence intensity, and quantifying the level of endotoxin in the sample based on a measured value obtained in the luminescence measuring step, the method enabling endotoxin to be simply and quickly measured at a level that cannot be detected in conventional methods for endotoxin measurement, without use of any dedicated measuring device.