PURPOSE: To provide a position detector which can achieve map matching even if map matching cannot be achieved under normal conditions.

CONSTITUTION: When a temporary set position where a vehicle could actually exist is set (S3), it is determined whether or not the linear distance L between the temporary set position and the estimated position of the vehicle as determined by the use of self-contained navigation is less than a threshold value LTH. (S5). When the linear distance L is determined as being not less than the threshold value LTH, that is, when errors in the estimated position of the vehicle are accumulated with the lapse of time and map matching cannot be achieved, it is determined whether or not the linear distance L' between the GPS position of the vehicle and the temporary set position is less than a second threshold value LTH (S8). When the linear distance L' is determined as being less than the second threshold value LTH', a temporary matching mode in which a car mark indicating the current position of the vehicle is displayed in the temporary set position is selected (S6). Map matching is thus achieved.