An object is to increase the amount of ions capable of entering and leaving a positive electrode active material in an ion battery so that the capacity of the battery is increased. When a solid solution including alkali metal oxide having electrical conductivity less than or equal to 10−10 S/cm and including alkali metal with a valence of 2 or more, and alkali metal oxide having electrical conductivity greater than or equal to 1×10−6 S/cm and less than or equal to 3×10−6 S/cm is used as a positive electrode active material in an ion battery, the amount of ions capable of entering and leaving the positive electrode active material is increased, so that the capacity of the battery is increased.