PURPOSE: To enable determination of accurate endotoxin concentration by deactivating reaction preventing factors and facilitating factors in solution by means of dilution heating or perchloric acid treatment and by creating a calibration curve with influence of various kinds of factors in the solution offset.

CONSTITUTION: Change in time required to be gel is observed in a nethelometric time analysis method as a measurement method. Time T0 required to be gel as to a specimen to be determined is measured from time change data of a ratio in transmitted light amounts. Then time periods required for specimen with endotoxin solution of known concentrations added to be gel are measured, which refer to T1, T2... Tn (n≥2). The solution concentration is adjusted to solution concentration C1, C2... Cn by adding the endotoxin solution of known concentrations to the specimen, and calibration curves with time T1, T2... Tn (n≥2) required to be gel which is measured in respective solution are created, while a concentration C0 is obtained from time T0 required to be gel based on the calibration curves. Then, by using C1+C0, C2+C0,... Cn+C0 wherein C0 is added to each of C1, C2... Cn and T1, T2... Tn to obtain a calibration curve again, operation for recalculating C0 from T0 is performed.