PURPOSE: To perform complete and sure heat sterilizing of liquid food by directly mixing steam, measuring a passing time of heated liquid food in a flowing path kept at a high temperature and discharging liquid food having the passing time longer than a fixed time.

CONSTITUTION: At first, liquid food is transferred to a steam-mixing device 2 by a feed pump and steam from a steam control valve is directly mixed to heat a fixed sterilizing temperature. Successively, said liquid food is passed through a flowing path kept at a high temperature in a state of kept at a sterilizing temperature and simultaneously passing time of the fluid food in the flowing path 9 is measured. At last, liquid food having said passing time attained to a time longer than a fixed time is discharged to a heat sterilized food-preserving part C.