Base wraps 21a and 31a of a fixed scroll 20 and orbiting scroll 30 both have spiral lengths that satisfy specified compression ratio in low speed drive region. Spiral shaped extension wraps 21b and 31b are provided for at least one of the base wraps 21a and 31a, extended from the spiral end position thereof, and a displacement is effected in the direction in which the thickness of the wraps 21 and 31 becomes thinner, at least at either the inner walls of the extension wraps 21b and 31b or at the outer walls of the wraps 21 and 31 in opposition thereto, making the interrelationship greater.