An electronic control device comprising: a housing having conductivity; a circuit board that is arranged in the housing and has a first electronic component mounted on a mounting surface; an external connection portion that is electrically connected to the circuit board and is connectable to a device outside the housing; and a duplexing member that has conductivity and is arranged between an inner surface of the housing and the mounting surface of the circuit board, in which the duplexing member forms a cutout portion to avoid contact between the duplexing member and the first electronic component, and further forms a partition portion that partitions the external connection portion between the housing and the circuit board.