An information processing device comprising: a storage unit including a first memory area (5_1) for storing a first data, a second memory area (5_2) for storing a second data, a third memory area (5_3) for storing a valid instruction flag indicating which of the first data and the second data is valid; a control unit for controlling erase and writing to the storage unit; and a usage data determination unit for determining which of the first data and the second data is valid and to be used, characterised in that the control unit is configured to store, in the storage unit, a first update status flag (8_1) indicating an update status of the first data, a second update status flag (8_2) indicating an update status of the second data, and a third update status flag (8_3) indicating an update status of the valid instruction flag, wherein the first update status flag is a flag indicating whether or not erase processing of the first memory area (5_1) performed for updating the first data has been started or completed, the second update status flag is a flag indicating whether or not erase processing of the second memory area (5_2) performed for updating the second data has been started or completed, and the third update status flag is a flag indicating whether or not erase processing of the third memory area (5_3) performed for updating the valid indication flag has been started or completed, and wherein the usage data determination unit is configured to determine which of the first data and the second data is valid when a determination using the valid instruction flag is impossible, by checking which of predefined reference patterns a pattern of values of the first, second and third update status flags (8_1, 8_2, 8_3) matches. a storage unit including a first memory area (5_1) for storing a first data, a second memory area (5_2) for storing a second data, a third memory area (5_3) for storing a valid instruction flag indicating which of the first data and the second data is valid; a control unit for controlling erase and writing to the storage unit; and a usage data determination unit for determining which of the first data and the second data is valid and to be used, characterised in that the control unit is configured to store, in the storage unit, a first update status flag (8_1) indicating an update status of the first data, a second update status flag (8_2) indicating an update status of the second data, and a third update status flag (8_3) indicating an update status of the valid instruction flag, wherein the first update status flag is a flag indicating whether or not erase processing of the first memory area (5_1) performed for updating the first data has been started or completed, the second update status flag is a flag indicating whether or not erase processing of the second memory area (5_2) performed for updating the second data has been started or completed, and the third update status flag is a flag indicating whether or not erase processing of the third memory area (5_3) performed for updating the valid indication flag has been started or completed, and wherein the usage data determination unit is configured to determine which of the first data and the second data is valid when a determination using the valid instruction flag is impossible, by checking which of predefined reference patterns a pattern of values of the first, second and third update status flags (8_1, 8_2, 8_3) matches.