In the method for manufacturing a SIMOX wafer, oxygen ions are implanted into a silicon wafer, then the silicon wafer is subjected to a prescribed heat treatment so as to form a buried oxide layer in the silicon wafer. The prescribed heat treatment includes: a step of ramping up a temperature of the silicon wafer in a low oxygen partial pressure gas atmosphere having an oxygen partial pressure ratio of less than 5%; either or both of a step of oxidizing the silicon wafer in a high oxygen partial pressure gas atmosphere having an oxygen partial pressure ratio of 5% or more and a step of annealing the silicon wafer in a low oxygen partial pressure gas atmosphere having an oxygen partial pressure ratio of less than 5%; and a step of ramping down the temperature of the silicon wafer in a low oxygen partial pressure gas atmosphere having an oxygen partial pressure ratio of less than 5%. A hydrogen chloride gas is mixed with the low oxygen partial pressure gas having an oxygen partial pressure ratio of less than 5% in at least one step from among the ramp-up step, the anneal step and the ramp-down step.