An apparatus (1) for continuously forming a continuous element (2) made of expanded plastic material comprises: a loading section (8) of a loose mass of granules made of expandable plastic material; a sintering section (16) of the continuous element (2) made of expanded plastic material extending downstream of the loading section (8); a stabilizing section (17) of the continuous element (2) made of expanded plastic material extending downstream of the sintering section (16); a forming runnel (18) extending along the sintering (16) and stabilizing (17) sections and defined between a pair of lateral walls (19, 20) and a pair of gas permeable conveyor belts (21, 22), respectively lower and upper, supported by respective supporting tracks (23, 24). According to the invention, at least one of the supporting tracks (23, 24) of the gas permeable conveyor belts (21, 22) comprises at least one portion extending in the sintering section (16) and angularly tiltable with respect to a transversal centerline plane (.pi.) of the forming tunnel (18) for varying the height of the tunnel (18) in a longitudinal direction. In this way, it is advantageously possible to adjust in an optimal manner both the expansion degree of the granules and their mutual welding to provide a compact structure having the desired homogeneous density and the desired homogeneous and low coefficient of heat conductivity.