A resistance generating device for a drive unit of an opening and closing member of a vehicle includes a transmission member transmitting a rotary motion to a rotary body, and a fixing member provided to form a clearance between the fixing member and the transmission member in a radial direction of a rotary axis of the transmission member. One of the fixing member and the transmission member includes a magnet and the other includes a magnetic body. The magnet is disposed to face the magnetic body so that the facing position facing each other is changed as the transmission member rotates. The magnet is disposed to face the magnetic body so that magnetic poles at the facing position alternate as the transmission member rotates. The magnetic body and the magnet generate a magnetic force therebetween, the magnetic force generating a resistance against an opening and closing movement of the opening and closing member.