A software update device configured to control software update of an electronic control unit (13a, 13b, 13c, 13d) mounted on a vehicle, the software update device comprising one or more processors configured: to download update data of software of the electronic control unit (13a, 13b, 13c, 13d) from a server; to transmit a validity confirmation request for confirming whether the update data is valid to the server before the update data is installed, to receive a confirmation result indicating whether the update data is valid from the server; to control the software update of the electronic control unit (13a, 13b, 13c, 13d); and not to install the update data when the one or more processors determine that the update data is invalid based on the confirmation result received by the one or more processors; to install the update data when the update data is determined to be valid based on the confirmation result received by the one or more processors; characterised in that the one or more processors are configured: to transmit a second validity confirmation request for confirming whether the update data is valid to the server after the update data is installed and before the update that is installed data is activated; to receive a second confirmation result, for the second validity confirmation request, indicating whether the update data is valid from the server; to set previous software before the update data is installed as the software to be executed by the electronic control unit (13a, 13b, 13c, 13d) in which the update data is installed when the one or more processors determine that the update data is invalid based on the second confirmation result received by the one or more processors; and to activate the software after the update data is installed, when the update data is determined to be valid based on the second confirmation result received by the one or more processors. to download update data of software of the electronic control unit (13a, 13b, 13c, 13d) from a server; to transmit a validity confirmation request for confirming whether the update data is valid to the server before the update data is installed, to receive a confirmation result indicating whether the update data is valid from the server; to control the software update of the electronic control unit (13a, 13b, 13c, 13d); and not to install the update data when the one or more processors determine that the update data is invalid based on the confirmation result received by the one or more processors; to install the update data when the update data is determined to be valid based on the confirmation result received by the one or more processors; characterised in that the one or more processors are configured: to transmit a second validity confirmation request for confirming whether the update data is valid to the server after the update data is installed and before the update that is installed data is activated; to receive a second confirmation result, for the second validity confirmation request, indicating whether the update data is valid from the server; to set previous software before the update data is installed as the software to be executed by the electronic control unit (13a, 13b, 13c, 13d) in which the update data is installed when the one or more processors determine that the update data is invalid based on the second confirmation result received by the one or more processors; and to activate the software after the update data is installed, when the update data is determined to be valid based on the second confirmation result received by the one or more processors. to transmit a second validity confirmation request for confirming whether the update data is valid to the server after the update data is installed and before the update that is installed data is activated; to receive a second confirmation result, for the second validity confirmation request, indicating whether the update data is valid from the server; to set previous software before the update data is installed as the software to be executed by the electronic control unit (13a, 13b, 13c, 13d) in which the update data is installed when the one or more processors determine that the update data is invalid based on the second confirmation result received by the one or more processors; and to activate the software after the update data is installed, when the update data is determined to be valid based on the second confirmation result received by the one or more processors.