A method and a slipform device for casting T beams or concrete elements having one or more longitudinal hollow cores. The slipform apparatus, which is movable in relation to the bed (2), comprises forming means (10) for producing cores. At the first stage, fresh concrete is fed into the slab part beneath the cores and, at the second stage, fresh concrete is fed between and onto the forming means. A plate (16, 20) is mounted on the bed, at the front and the back end of the element, which plate comprises openings (17) to receive the forming means. Above the openings, slots (18) are provided to receive the supports (11) of the forming means.