A novel broadbeam, planar leaky-wave antenna with radiation at broadside in the millimeter-wave frequency range using substrate-integrated waveguide technology is proposed. By feeding the antenna at the middle point along its length and by working at the splitting condition, a bi-directional traveling leaky-wave gives rise to a beam at the broadside direction in the far-field. The design is validated with full-wave simulations showing that a half power beamwidth of 30° and 130°, in the H- and the E-planes, are respectively possible for a 375MHz bandwidth (1% fractional bandwidth) around 24GHz. In this frequency range, realized gain values are above 8dBi.