A photo detection device, which is disposed in a vehicle, includes a first photo detection element that detects light and a first signal processing unit that performs a predetermined process based on a detection result of the first photo detection element. The photo detection device also includes a first board on which the first photo detection element is mounted and a second board on which the first signal processing unit is mounted. Additionally, the photo detection device includes a holding member that holds the first board and the second board and a housing that accommodates the holding member. The holding member holds the first board and the second board such that the second board is arranged in a second plane different from a first plane in which the first board is arranged.