An apparatus for detecting a lead distance between a user vehicle and a target vehicle includes first and second position sensitive devices (PSD) arranged in the user vehicle orthogonal to the distance and each having first and second light receivers configured to receive light emitted from the target vehicle, in which the first and second PSDs output voltage corresponding intensity of light received by the first and second light receivers, and a controller configured to receive voltages output from the first and second PSDs and calculate the distance using Equation 1: L=(KD/d)/(v1−v2).