A horn speaker system includes a dome-shaped diaphragm, a coil bobbin, a phase equalizer, and a magnetic circuit. The coil bobbin is integral with the dome-shaped diaphragm, and a voice coil is wound on the coil bobbin near a free end thereof. The magnetic circuit coacts with the voice coil for producing forces to actuate the dome-shaped diaphragm. The magnetic circuit includes a cylindrical magnet, a first annular yoke attracted to one end of the cylindrical magnet and having a central opening, a plate cooperating with the first yoke in defining a gap in which the coil bobbin is inserted, and a second yoke attracted to an opposite end of the magnet. The magnet, the first yoke, the first plate, and the second yoke jointly make up a closed magnetic circuit. The phase equalizer serves to keep sounds outputted from the dome-shaped diaphragm in phase. The phase equalizer is mounted on the first yoke and has a plurality of slits defined therein. The magnet, the first yoke, the phase equalizer, and the dome-shaped diaphragm are stacked coaxially with each other.