To provide a method for operating a steam blowing-in type direct heating sterilizer, by which a sterilization time can be monitored to improve the reliability of the sterilizer.

This method for operating the steam blowing-in type direct heating sterilizer, comprising directly blowing steam in a liquid to be sterilized to heat the liquid and flowing the heated liquid in a holding pipe to hold the liquid in the holding pipe for a prescribed holding time at a prescribed holding temperature, is characterized by changing a part of the holding pipe into a transparent pipe and then measuring the flow speed of the liquid in the holding pipe with an optical fiber type laser flow speed meter through the transparent pipe to monitor whether or not the flow speed is within a range capable of holding the liquid for the prescribed holding time in the holding pipe.