An inter-vehicle distance estimation method using a sensor that detects positional information of objects around a vehicle , and a processing circuit that estimates repetitively at predetermined times inter-vehicle distances between a plurality of tracked vehicles traveling in the same traffic lane and detected based on the positional information, comprising: causing the processing circuitry to detect observable objects around the vehicle and to determine attribute information including positions and speeds of the observable objects ; characterized by the plurality of tracked vehicles being formed of a plurality of observable objects and a plurality of unobserved objects ; and causing the processing circuit to estimate a screened area screened by the observable objects from the sensor and two unscreened areas not screened by the observable objects from the sensor and sandwiching the screened area ; and causing the processing circuit to, using positions and speeds of two of the tracked vehicles traveling in a same traffic lane respectively within the two unscreened areas , estimate a speed of one of the tracked vehicles traveling in the same traffic lane within the screened area ; causing a screen determination unit comprised in the processing circuit to determine whether each object is present in any one of the screened area and the unscreened area , wherein an object determined by the screen determination unit as not being screened is an observable object , whereas an object determined by the screen determination unit as being screened is an unobserved object ; and causing the processing circuit to estimate a current position of the one of the tracked vehicles traveling in the same traffic lane within the screened area by calculating an amount of change in the distance between the tracked vehicles from the speeds estimated at a previous time of the predetermined times and a cycle duration and to estimate the inter-vehicle distances from the two tracked vehicles to the one of the tracked vehicles traveling in the same traffic lane within the screened area . causing the processing circuitry to detect observable objects around the vehicle and to determine attribute information including positions and speeds of the observable objects ; characterized by the plurality of tracked vehicles being formed of a plurality of observable objects and a plurality of unobserved objects ; and causing the processing circuit to estimate a screened area screened by the observable objects from the sensor and two unscreened areas not screened by the observable objects from the sensor and sandwiching the screened area ; and causing the processing circuit to, using positions and speeds of two of the tracked vehicles traveling in a same traffic lane respectively within the two unscreened areas , estimate a speed of one of the tracked vehicles traveling in the same traffic lane within the screened area ; causing a screen determination unit comprised in the processing circuit to determine whether each object is present in any one of the screened area and the unscreened area , wherein an object determined by the screen determination unit as not being screened is an observable object , whereas an object determined by the screen determination unit as being screened is an unobserved object ; and causing the processing circuit to estimate a current position of the one of the tracked vehicles traveling in the same traffic lane within the screened area by calculating an amount of change in the distance between the tracked vehicles from the speeds estimated at a previous time of the predetermined times and a cycle duration and to estimate the inter-vehicle distances from the two tracked vehicles to the one of the tracked vehicles traveling in the same traffic lane within the screened area . <EOS>