1. A radar apparatus for use in a vehicle comprising: a transmitting/receiving means configured to transmit a radar wave in accordance with a transmission signal frequency-modulated so as to linearly vary in frequency with time at every measurement cycle, receive the radar wave reflected from a target as a reflected wave, output a reception signal in accordance with the received reflected wave, and generate a beat signal by mixing the transmission signal with the reception signal; a frequency analyzing means configured to calculate power spectrum indicative of intensity of each frequency component of the beat signal each time the transmitting/receiving means generates the beat signal; a target candidate detecting means for detecting a peak frequency at which the intensity of the power spectrum peaks as a target candidate and calculating a position and a speed of the target candidate each time the frequency analyzing means calculates the power spectrum; a road shape recognizing means configured to sequentially connect, along a predetermined direction, the target candidates detected to be stationary and present within a first reference distance from one of the target candidates set as a reference target candidate by the target candidate detecting means for one or more measurement cycles in order to prepare a group of the target candidates sequentially connected, and recognize an area formed by the sequentially connected target candidates as an edge of a road on which the vehicle runs; an object recognizing means configured to determine, for each of the target candidates detected to be moving by the target candidate detecting means, that the target candidate is a moving object if a position of the target candidate detected in the present measurement cycle continues to be within a predetermined second reference distance from a predicted position estimated from a position of the target candidate detected in the previous measurement cycle for a predetermined number of the measurement cycles; a cruise environment estimating means configured to estimate that a cruise environment representing a road environment of the vehicle is a closed space where space above the vehicle is closed if an integral value of intensity of the power spectrum calculated by the frequency analyzing means for a predetermined frequency range is larger than a predetermined threshold value, and otherwise is an open space where space above the vehicle is open; and a reference distance correcting means configured to shorten at least one of the first and second reference distances when the cruise environment is estimated to be a closed space by the cruise environment estimating means compared to when the cruise environment is estimated to be an open space by the cruise environment estimating means.