This in-vehicle control device includes: a cooling period setting unit that sets a timing prior to the start timing of a predicted knock occurrence period as the increasing timing of a cooling amount of a combustion room, and sets a timing prior to an end timing of the predicted knock occurrence period as the decreasing timing of the cooling amount of the combustion room; and a cooling amount change unit that changes the cooling amount of the combustion room by a cooling mechanism on the basis of the set increasing and decreasing timings and a target cooling amount set by a target cooling amount setting unit.