A magnesium oxide powder containing magnesium oxide particles, wherein an average sphericity of a magnesium oxide particle having a projected area equivalent circle diameter of a particle cross section by microscopy of 10 μm or more among the magnesium oxide particles is 0.73 or more and 0.95 or less, a proportion of a magnesium oxide particle having a sphericity of 0.65 or less among the magnesium oxide particles is 20% or less on a number basis, and a proportion of a magnesium oxide particle having a sphericity of 0.55 or less among the magnesium oxide particles is 15% or less on a number basis.