A fuel cell system comprising: a fuel cell configured to generate electric power using fuel and an oxidizing agent; a water passage which is a passage of water related to an operation of said fuel cell; an electric heater configured to heat said water passage; a first abnormality detector configured to detect first abnormalities including abnormalities which are related to leakage of a combustible gas and disturb safety of the fuel cell system by activating the heater while these abnormalities are occurring; a second abnormality detector configured to detect second abnormalities which are different from the first abnormalities and do not disturb the safety of the fuel cell system even by activating the heater while these abnormalities are occurring; and a controller, said controller being configured to stop an operation of the fuel cell system when the first abnormality is detected by said first abnormality detector or when the second abnormality is detected by said second abnormality detector, wherein: said controller is configured to cause said electric heater to carry out an operation as an antifreezing operation in a case where the fuel cell system stops since the second abnormality is detected by said second abnormality detector; and said controller is configured not to cause said electric heater to carry out the operation as the antifreezing operation in a case where the fuel cell system stops since the first abnormality is detected by said first abnormality detector. a fuel cell configured to generate electric power using fuel and an oxidizing agent; a water passage which is a passage of water related to an operation of said fuel cell; an electric heater configured to heat said water passage; a first abnormality detector configured to detect first abnormalities including abnormalities which are related to leakage of a combustible gas and disturb safety of the fuel cell system by activating the heater while these abnormalities are occurring; a second abnormality detector configured to detect second abnormalities which are different from the first abnormalities and do not disturb the safety of the fuel cell system even by activating the heater while these abnormalities are occurring; and a controller, said controller being configured to stop an operation of the fuel cell system when the first abnormality is detected by said first abnormality detector or when the second abnormality is detected by said second abnormality detector, wherein: said controller is configured to cause said electric heater to carry out an operation as an antifreezing operation in a case where the fuel cell system stops since the second abnormality is detected by said second abnormality detector; and said controller is configured not to cause said electric heater to carry out the operation as the antifreezing operation in a case where the fuel cell system stops since the first abnormality is detected by said first abnormality detector. said controller is configured to cause said electric heater to carry out an operation as an antifreezing operation in a case where the fuel cell system stops since the second abnormality is detected by said second abnormality detector; and said controller is configured not to cause said electric heater to carry out the operation as the antifreezing operation in a case where the fuel cell system stops since the first abnormality is detected by said first abnormality detector.