There are provided: a solid polymer power generation or electrolysis method that does not require injection of energy from the outside and maintenance of a high temperature, and is capable of converting carbon dioxide to a useful hydrocarbon while producing energy, controlling the production amounts of the hydrocarbons or the like and a ratio sorted by kind of the hydrocarbons, improving utilization efficiency of a product, and simplifying equipment for separation and recovery; and a system for implementing the solid polymer power generation or electrolysis method. Carbon dioxide is supplied to the side of one electrode 111 of a reactor 110 having a membrane electrode assembly 113, hydrogen is supplied to the side of the other electrode 112, and the amounts of the hydrocarbons produced per unit time and the ratio sorted by kind of the hydrocarbons are changed by controlling a power generation voltage of the reactor 110.