A wire connection device includes an electrical wire, an insulation displacement terminal and a stress relaxation member. The electrical wire includes a conductor and an insulation coating on an outer periphery of the conductor. The insulation displacement terminal has a slot in which the electrical wire is fitted. The insulation displacement terminal is connected to the electrical wire by both side portions of the slot being in direct contact with the conductor and pressing the conductor elastically and plastically. The stress relaxation member is fitted in the slot of the insulation displacement terminal to suppress deterioration of a stress applied to the electrical wire.