A duct installation structure in which an air conditioning duct for sending warm air or cold air into a vehicle interior of an automobile is attached to a vehicle interior structure member and an outlet grill is attached to an outlet opening end portion of the air conditioning duct. A grill fixing portion to which the outlet grill is to be fixed is integrally molded with a duct main body portion at the outlet opening end portion of the air conditioning duct. The outlet grill is directly fixed to the grill fixing portion. The outlet opening end portion of the air conditioning duct including, at one point near its center in a width direction, a fastening fixing portion that is fastened and fixed to the vehicle interior structure member and including, at its opposite ends in the width direction, a pair of contact portions for coming in contact with the vehicle interior structure member.