Abstract: An annealing separator, by being applied to a steel sheet, enables retention of a good shape of a coil after annealing obtained by winding the steel sheet with the annealing separator applied thereto into a coil which is then annealed at a high temperature exceeding 1000° C. A powder used as the annealing separator: contains magnesium oxide as a main component and B: 0.020 mass % or more and 0.200 mass % or less, SO3: 0.030 mass % or more and 2.000 mass % or less, and P2O3: 0.050 mass % or more and 1.000 mass % or less; and contains 0.2 mass % or more and 5.0 mass % or less of particles having a particle size of more than 45 ?m and 75 ?m or less. The particles having this particle size contain (boron) B: 0.002 mass % or more and less than 0.020 mass %, SO3: less than 0.030 mass %, and P2O3: less than 0.050 mass %.

Abstract: Provided is a powder for annealing separator that is used for a grain-oriented electrical steel sheet and enables obtainment of a grain-oriented electrical steel sheet having excellent magnetic properties as a result of favorable purification of impurities in steel. The powder for annealing separator contains magnesium oxide as a main component, and has a nitrogen gas permeability of 0.020 cm/s or more and 0.50 cm/s or less in a state of being applied between coiled steel sheets before start of secondary recrystallization.