1. A sign recognition device comprising: a sign effective range data recording unit for storing conditions for an effective range shown by a sign and an auxiliary sign; a sign recognition unit for recognizing a sign and an auxiliary sign by using a captured image of a roadway in front of a vehicle; and a sign effective range determining unit for determining whether or not the vehicle is staying in an effective range specified by the sign and the auxiliary sign which have been recognized by said sign recognition unit by using information about the vehicle, map information, and conditions for said effective range which are stored in said sign effective range data recording unit, wherein said sign effective range determining unit includes a start point position determining unit for determining a position of a start point of the effective range specified by said sign and said auxiliary sign which have been recognized by said sign recognition unit, and for, when the vehicle has entered the effective range specified by said sign and said auxiliary sign, storing a result of the recognition of said sign and said auxiliary sign in an effective sign storage buffer, and an end point position determining unit for determining a position of an end point of the effective range specified by said sign and said auxiliary sign whose recognition result is stored in said effective sign storage buffer, and for, when the vehicle has deviated from the effective range specified by said sign and said auxiliary sign, deleting the result of the recognition of said sign and said auxiliary sign from said effective sign storage buffer.