A fuel cell system comprising: a fuel cell configured to generate power through electrochemical reaction between fuel gas and oxidant gas; a fuel gas supply path for supplying the fuel gas to an anode of the fuel cell; an offgas discharge path for discharging offgas from the anode of the fuel cell; a recycle gas path that is bifurcated from the offgas discharge path at a bifurcation and joins the fuel gas supply path; a flow rate adjuster that is provided on the recycle gas path and configured to adjust a flow rate of gas flowing through the recycle gas path; a purge valve provided on the offgas discharge path downstream of the bifurcation; and a controller; characterized in that the controller is configured to set an upper limit value of the flow rate of the flow rate adjuster, maintain an internal pressure of the offgas discharge path upstream of the purge valve to be higher than atmospheric pressure, with the purge valve being closed, by controlling the flow rate adjuster to adjust a value of the flow rate to the upper limit value or less, and after the internal pressure of the off gas discharge path becomes higher than the atmospheric pressure, the controller is configured to control the purge valve to open, wherein the flow rate adjuster is a booster that is configured to perform pressurized transfer of the offgas to the fuel gas supply path, the fuel cell system further comprises a pressure sensor that is configured to sense a supply pressure of the fuel gas, wherein an upper limit value of an output of the booster is set based on the supply pressure sensed by the pressure sensor, wherein the upper limit value of the output of the booster is set to have a positive correlation with a change of the supply pressure sensed by the pressure sensor. a fuel cell configured to generate power through electrochemical reaction between fuel gas and oxidant gas; a fuel gas supply path for supplying the fuel gas to an anode of the fuel cell; an offgas discharge path for discharging offgas from the anode of the fuel cell; a recycle gas path that is bifurcated from the offgas discharge path at a bifurcation and joins the fuel gas supply path; a flow rate adjuster that is provided on the recycle gas path and configured to adjust a flow rate of gas flowing through the recycle gas path; a purge valve provided on the offgas discharge path downstream of the bifurcation; and a controller; characterized in that the controller is configured to set an upper limit value of the flow rate of the flow rate adjuster, maintain an internal pressure of the offgas discharge path upstream of the purge valve to be higher than atmospheric pressure, with the purge valve being closed, by controlling the flow rate adjuster to adjust a value of the flow rate to the upper limit value or less, and after the internal pressure of the off gas discharge path becomes higher than the atmospheric pressure, the controller is configured to control the purge valve to open, set an upper limit value of the flow rate of the flow rate adjuster, maintain an internal pressure of the offgas discharge path upstream of the purge valve to be higher than atmospheric pressure, with the purge valve being closed, by controlling the flow rate adjuster to adjust a value of the flow rate to the upper limit value or less, and after the internal pressure of the off gas discharge path becomes higher than the atmospheric pressure, the controller is configured to control the purge valve to open, wherein the flow rate adjuster is a booster that is configured to perform pressurized transfer of the offgas to the fuel gas supply path, the fuel cell system further comprises a pressure sensor that is configured to sense a supply pressure of the fuel gas, wherein an upper limit value of an output of the booster is set based on the supply pressure sensed by the pressure sensor, wherein the upper limit value of the output of the booster is set to have a positive correlation with a change of the supply pressure sensed by the pressure sensor.