A state of charge estimation device for estimating a state of charge of iron phosphate lithium ion batteries includes a voltage measurement component configured to measure a voltage across the iron phosphate lithium ion batteries, a current measurement component configured to measure a current flowing through the iron phosphate lithium ion batteries, and a controller component configured to, when an estimation of the state of charge of the iron phosphate lithium ion batteries based on an accumulated amount of currents measured by the current measurement component is defined as a current accumulation method and an estimation of the state of charge of the iron phosphate lithium ion batteries based on an open circuit voltage (OCV) of the iron phosphate lithium ion batteries measured by the voltage measurement component is defined as an OCV method, permit the estimation of the state of charge by the OCV method.