The distance measuring device includes a light source (1), a light-receiving sensor (2), a timing controller (5), a distance calculator (6), and a delay controller (8). The timing controller (5) outputs a modulation signal and plural reference timing signals. The modulation signal is a square wave signal having high and low level periods appearing alternately. Each of the high and low level periods has its length randomly selected from integral multiples of a predetermined unit time period. The reference timing signals include a signal having the same waveform as that of the modulation signal and a signal having the same waveform as that of the inverted modulation signal. The light source (1) varies an intensity of the light in concordance with the modulation signal. The delay controller (8) delays the plural reference timing signals by the delay period (Td) to create plural timing signals respectively. The light-receiving sensor (2) accumulates the electric charges generated within the reception time period, with regard to each of the timing signals. The distance calculator (6) calculates the time difference (τ) from amounts of the electric charges respectively associated with the timing signals, and calculates a distance (L) to the target (3) on the basis of the time difference (τ) and the delay period (Td).