A circuit breaker is comprised of a make-and-break mechanism part disposed between frame plates opposite to each other; a base in which a breaking portion including cross bar, moving contact, fixed contact and arc-extinguishing device, and an over-current trip device are contained; a middle base, in which a partition wall separating the breaking portion from the make-and-break mechanism part, andthe breaking portion from the over-current trip device, is provided; and a cover for covering the base and middle base. A frame of the make-and-break mechanism part is secured to the base through themiddle base, and the secured part of the frame and the inner bottom face of the base are disposed in the proximity.