The present inventoin provides a reagent for prothrombin time measurement, method for production thereof, and method for measurement of prothrombin time. Disclosed is a reagent for prothrombin time measurement, containing a tissue factor and a surfactant, wherein the value of formula (I), which is represented by (surfactant amount ([Mu]mol))/(total protein amount ([Mu]g)), is 0.013 to 0.05 [Mu]mol/[Mu]g. The production method comprises: the step of the cooling and drying of water solution containing tissue factors and surface active agent with 0.013 to 0.05 [Mu]mol/[Mu]g (surfactant amount ([Mu]mol))/(total protein amount ([Mu]g)).