Temperature maintenance and/or possible heating apparatus for long products is disposed between a rolling line (22) and a casting machine (11). The apparatus comprises a first furnace (36), a second furnace (37) and a transit tunnel (38). The first furnace (36) is provided with a first introduction rollerway (41) to introduce the bloom segments, a removal rollerway (43), and transverse transfer devices (25, 26) to transfer the bloom segments from the first introduction rollerway (41) to the removal rollerway (43). The second furnace (37) is disposed upstream with respect to said first furnace (36), provided with a second introduction rollerway (42) to introduce the bloom segments, disposed aligned with said second casting line (21b), an exit rollerway (44) aligned with the removal rollerway (43) and suitable to convey the bloom segments toward said removal rollerway (43) and transverse transfer devices (25, 26) suitable to transfer the bloom segments from the second introduction rollerway (42) toward the exit rollerway (44). The transit tunnel (38) is located aligned with said first casting line (21a), adjacent and parallel to the longitudinal extension of the second furnace (37) and upstream of said first furnace (36), conformed to transport the cast bloom segments from said first casting line (21a) toward the first introduction rollerway (41) of the first furnace (36).