A positive electrode for a lithium ion secondary battery, the positive electrode comprising: a positive electrode current collector; and a positive electrode mixture layer that is provided on at least one side of the positive electrode current collector, the positive electrode mixture layer including: a positive electrode active material layer, and an undercoat layer formed between the positive electrode current collector and the positive electrode active material layer, the undercoat layer containing: a conductive auxiliary, a binder, a thermally expandable microcapsule having a maximum volume expansion temperature of from 70°C to 180°C, said thermally expandable microcapsule being configured to enclose or encapsulate a volatile expanding agent in an outer shell which is composed of a thermoplastic resin, and a polymer particle having a softening point of from 70°C to 150°C, the softening point of the polymer particle is lower than the maximum volume expansion temperature of the thermally expandable microcapsule, and the polymer particle has an average particle size of from 0.1 to 5 μm, the average particle size being determined by the method provided in the description. a positive electrode current collector; and a positive electrode mixture layer that is provided on at least one side of the positive electrode current collector, the positive electrode mixture layer including: a positive electrode active material layer, and an undercoat layer formed between the positive electrode current collector and the positive electrode active material layer, the undercoat layer containing: a conductive auxiliary, a binder, a thermally expandable microcapsule having a maximum volume expansion temperature of from 70°C to 180°C, said thermally expandable microcapsule being configured to enclose or encapsulate a volatile expanding agent in an outer shell which is composed of a thermoplastic resin, and a polymer particle having a softening point of from 70°C to 150°C, the softening point of the polymer particle is lower than the maximum volume expansion temperature of the thermally expandable microcapsule, and the polymer particle has an average particle size of from 0.1 to 5 μm, the average particle size being determined by the method provided in the description. a positive electrode active material layer, and an undercoat layer formed between the positive electrode current collector and the positive electrode active material layer, the undercoat layer containing: a conductive auxiliary, a binder, a thermally expandable microcapsule having a maximum volume expansion temperature of from 70°C to 180°C, said thermally expandable microcapsule being configured to enclose or encapsulate a volatile expanding agent in an outer shell which is composed of a thermoplastic resin, and a polymer particle having a softening point of from 70°C to 150°C, the softening point of the polymer particle is lower than the maximum volume expansion temperature of the thermally expandable microcapsule, and the polymer particle has an average particle size of from 0.1 to 5 μm, the average particle size being determined by the method provided in the description. a conductive auxiliary, a binder, a thermally expandable microcapsule having a maximum volume expansion temperature of from 70°C to 180°C, said thermally expandable microcapsule being configured to enclose or encapsulate a volatile expanding agent in an outer shell which is composed of a thermoplastic resin, and a polymer particle having a softening point of from 70°C to 150°C, the softening point of the polymer particle is lower than the maximum volume expansion temperature of the thermally expandable microcapsule, and the polymer particle has an average particle size of from 0.1 to 5 μm, the average particle size being determined by the method provided in the description.