PURPOSE: To obtain the subject powder having uniform quality and sharp particle size distribution while suppressing the pulverization by transferring a raw silica powder through a vertical mobile layer furnace from the top downward and baking the powder while passing a small amount of a gas from the bottom of the furnace.

CONSTITUTION: A high-purity silica gel powder having particle diameter of 100-800μm, a bulk density of 0.6-0.7g/cm3, a metal content of ≤1ppm and a water content of ≤10wt.% is supplied from an inlet port 3 through a valve 2 into the main body of a furnace 6 to form a mobile powder layer continuously moving from the top downward (the moving speed is 10-50cm/hr). A small amount of a gas (e.g. air) is supplied from an inlet port 11 through a gas-diffuser 9 into the furnace, transferred upward through the gel particles and discharged from a discharging port 4 (the linear velocity of the gas is 10-3000cm/min). The inner wall surface of the furnace 6 is made of an abrasion-resistant ceramic material and maintained to 1000-1300°C with a heater 1. The raw material powder is baked in the course of passing through the furnace spending 1-30hr, cooled with a coolant flowing from the inlet 8 to the outlet 7 and discharged from the system by a feeder 10.