A gearbox bracket and a power seat sliding device are provided at low cost. A base part 351 includes a main body part 353 configured to support a gearbox 331, and a first protruding part 355 and a second protruding part 357 continuously connected to the main body part 353 and protruding in the direction of the output shaft. A side portion of the first protruding part 355 and a side portion of a first mounting part 371 are connected by a first vertical wall part 361 and a second vertical wall part 363. A side portion of the second protruding part 357 and a side portion of a second mounting part 373 are connected by a third vertical wall part 365 and a fourth vertical wall part 367.