1. A solar radiation detection unit for a vehicle, provided for a vehicle equipped with a navigation system which detects a present position and a travel direction of the vehicle, and guides the vehicle in travelling based on the detected present position and the detected travel direction of the vehicle, and road information including information of structures at least around a road previously stored, the solar radiation detection unit being for detecting intensity of solar radiation entering a vehicle compartment, comprising: solar position storage means in which solar position information representing a solar position which changes in accordance with the date and time is stored in advance; solar-radiation amount determining means for determining a basic solar-radiation amount receiving from the sun; solar-radiation direction calculating means which reads solar position information corresponding to the present date and time from the solar position storage means and the road information around the vehicle detected by the navigation system, and computes presence or absence of solar radiation entering the vehicle and an entering direction on the basis of the solar position information and the road information; and solar-radiation amount calculation means which determines the solar radiation amount entering the vehicle on the basis of calculation results of the solar-radiation direction calculating means and the solar-radiation basic amount of the solar-radiation amount means, wherein the solar-radiation amount calculation means corrects the solar radiation amount based on information unique to the vehicle.