An operation stop method of a fuel cell power generation system including a fuel generator connected to a raw material gas supply passage to receive raw material gas from a raw material gas supply device and connected to a water supply passage to receive water from a water supply device, and a fuel cell, the fuel cell including an electrolyte membrane, an anode, a cathode, a fuel gas channel through which a fuel gas is supplied to the anode and an oxidizing gas channel through which an oxidizing gas is supplied to the cathode, the fuel cell being configured to generate electric power by causing a reaction between the fuel gas supplied to the anode and the oxidizing gas supplied to the cathode, wherein said fuel gas is generated in the fuel generator and is supplied to the fuel gas channel of the fuel cell through a fuel gas supply passage, the operation stop method comprising: a step in which an output controller reduces an amount of electric power extracted from the fuel cell to be less than an amount of electric power previously generated by the fuel cell and to be in a range no less than a self-sustained operation-enabled power amount, and then stops supplying the electric power to an external load; a step in which an oxidizing gas supply device configured to supply the oxidizing gas to the oxidizing gas channel of the fuel cell through an oxidizing gas supply passage stops supplying the oxidizing gas, an open-close mechanism (74, 71) closes a passage upstream from the oxidizing gas channel, and after the passage upstream from the oxidizing gas channel is closed, the raw material gas supply device and the water supply device stop when a predetermined period has elapsed, during which period the oxidizing gas in the oxidizing gas channel is consumed by the fuel gas that has cross-leaked to the oxidizing gas channel through the electrolyte membrane; and a step in which the open-close mechanism closes a passage upstream from the fuel gas channel and a passage downstream from the fuel gas channel after the step. a step in which an output controller reduces an amount of electric power extracted from the fuel cell to be less than an amount of electric power previously generated by the fuel cell and to be in a range no less than a self-sustained operation-enabled power amount, and then stops supplying the electric power to an external load; a step in which an oxidizing gas supply device configured to supply the oxidizing gas to the oxidizing gas channel of the fuel cell through an oxidizing gas supply passage stops supplying the oxidizing gas, an open-close mechanism (74, 71) closes a passage upstream from the oxidizing gas channel, and after the passage upstream from the oxidizing gas channel is closed, the raw material gas supply device and the water supply device stop when a predetermined period has elapsed, during which period the oxidizing gas in the oxidizing gas channel is consumed by the fuel gas that has cross-leaked to the oxidizing gas channel through the electrolyte membrane; and a step in which the open-close mechanism closes a passage upstream from the fuel gas channel and a passage downstream from the fuel gas channel after the step.