An electronic control device, which comprises a nonvolatile memory (140, 160), wherein: - the nonvolatile memory (140, 160) has allocated two storage areas (A, B) that are exclusively switchable between an active state and an inactive state, - the storage area (A, B) in the active state is configured to be used to store a control program to be executed, - the storage area (A, B) in the inactive state is configured to be used as a reserved area for updating the control program, - the electronic control device is configured - such that, when the control program is not updated, arbitrary data is written in the storage area (A, B) in the inactive state, and - to determine whether a failure has occurred, and if it is determined that a failure has occurred, to write failure information in the storage area (A, B) in the inactive state, - the arbitrary data is failure information, and - the electronic control device is further configured such that, when the control program is updated: - the arbitrary data written in the storage area (A, B) in the inactive state is transmitted to an external device, and - an update program transmitted from the external device is written in the storage area (A, B) in the inactive state. which comprises a nonvolatile memory (140, 160), wherein: - the nonvolatile memory (140, 160) has allocated two storage areas (A, B) that are exclusively switchable between an active state and an inactive state, - the storage area (A, B) in the active state is configured to be used to store a control program to be executed, - the storage area (A, B) in the inactive state is configured to be used as a reserved area for updating the control program, - the electronic control device is configured - such that, when the control program is not updated, arbitrary data is written in the storage area (A, B) in the inactive state, and - to determine whether a failure has occurred, and if it is determined that a failure has occurred, to write failure information in the storage area (A, B) in the inactive state, - the arbitrary data is failure information, and - the electronic control device is further configured such that, when the control program is updated: - the arbitrary data written in the storage area (A, B) in the inactive state is transmitted to an external device, and - an update program transmitted from the external device is written in the storage area (A, B) in the inactive state. - the nonvolatile memory (140, 160) has allocated two storage areas (A, B) that are exclusively switchable between an active state and an inactive state, - the storage area (A, B) in the active state is configured to be used to store a control program to be executed, - the storage area (A, B) in the inactive state is configured to be used as a reserved area for updating the control program, - the electronic control device is configured - such that, when the control program is not updated, arbitrary data is written in the storage area (A, B) in the inactive state, and - to determine whether a failure has occurred, and if it is determined that a failure has occurred, to write failure information in the storage area (A, B) in the inactive state, - such that, when the control program is not updated, arbitrary data is written in the storage area (A, B) in the inactive state, and - to determine whether a failure has occurred, and if it is determined that a failure has occurred, to write failure information in the storage area (A, B) in the inactive state, - the arbitrary data is failure information, and - the electronic control device is further configured such that, when the control program is updated: - the arbitrary data written in the storage area (A, B) in the inactive state is transmitted to an external device, and - an update program transmitted from the external device is written in the storage area (A, B) in the inactive state. - the arbitrary data written in the storage area (A, B) in the inactive state is transmitted to an external device, and - an update program transmitted from the external device is written in the storage area (A, B) in the inactive state.