An energy storage device includes a positive electrode having a positive active material layer containing an active material in the form of particles. The positive active material layer contains primary particles of the active material and secondary particles formed by aggregation of a plurality of primary particles. The proportion of primary particles relative to all particles of the active material in the positive active material layer is 5% or more and 40% or less. An method for producing an energy storage device includes preparing a positive electrode having a positive active material layer by forming a positive active material layer from a composite containing at least secondary particles of an active material, and assembling an energy storage device using the prepared positive electrode. In the preparation of the positive electrode, the positive active material layer is pressed to deagglomerate some of the secondary particles into primary particles, and the proportion of primary particles relative to all particles of the active material in the positive active material layer is adjusted to 5% or more and 40% or less.