The present invention provides an electronic control unit casing that can be made more compact while maintaining the waterproof function. The electronic control unit casing (1) has: a circuit board (3), a housing (4) receiving the circuit board (3) and provided with an opening part at one end side of the circuit board (3), and an electrical connector (2) having a plurality of contacts (22) electrically connected with the circuit board (3) and of which a front end side is exposed to the outer side of the housing (4) from the opening part. The electrical connector (2) has a sealing part engaged with the opening part, the sealing part has an upper channel (24a) arranged at one side of the circuit board (3), and a lower channel (24b) arranged at the other side of the circuit board (3), the lower channel (24b) is offset closer to the front end side of the electrical connector (2) than the upper channel (24a).