A relative position detection device is used for a traveling system that allows a first and second vehicles to travel such that the first vehicle precedes and the second vehicle follows the first vehicle. The detecting device includes a sensor pair of a first and second sensors and an arithmetic unit. The first and second sensors are located at the same height on the right and left end portions of the front surface of the second vehicle. The first and second sensors emits transmission waves from their located positions in their front directions and receives their reflected waves. The arithmetic unit detects a direction of deviation between the first and second vehicles in the right and left direction on the basis of a difference between a first intensity of the reflected wave received by the first sensor and a second intensity of the reflected waves received by the second sensor.