To provide an outdoor unit for an air conditioner capable of sufficiently cooling a reactor mounted on an electric component module, and preventing propagation of vibration generating from the reactor.

A base plate 10a of the reactor 10 is fixed to a pedestal 11 composed of a damping steel plate and having a number of slot-shaped ventilating holes 11a by screws 13 through a cylindrical first spacer 12. A cylindrical second spacer 14 composed of an elastic material such as a rubber material and a soft resin material is vertically mounted in a cut formed on a peripheral edge of the pedestal 11, and screws 15 are inserted into insertion holes of the second spacer 14 and engaged with threaded holes formed on a chassis 9a to fix the pedestal 11 to the chassis 9a.