(a) receiving laser beams reflected from a target, and converting the same into a corresponding photocurrent signal, and converting the signal into a voltage signal;

(b) canceling a noise component from the voltage signal and converting the noise-cancelled voltage signal into range-finding data;

(c) sequentially storing the range-finding data;

(d) adding the stored range-finding data and previously processed and stored accumulated data, and storing results as accumulated data;

(e) detecting data exceeding a threshold value from among the accumulated data as target signals; and

(f) reading a target range based on the detected target signals, wherein

(a) through (d) are repeated N times, and the accumulated data in (e) are obtained by repeating (a) through (d) N times,

whrerin (b) further includes differentiating the voltage signal to cancel a voltage component that corresponds to the noise component and is superimposed on the voltage signal; and

filtering the differentiated signal with a predetermined frequency bandwidth identical to a frequency band of the target signal,

wherein the bandwidth satisfies 0.35/t_r wherein t_r is a rising time of a laser pulse, and a cut-off frequency satisfies ½τ wherein τ is a full width at half the maximum.