A method and an apparatus for web threading in a drying section of a paper machine or the like. A leader (24) and a so called run-down strip are formed by cutting in the web coming to the drying cylinder in the drying section. An underpressure box (18) is arranged at the opening gap between the drying cylinder (14) and the drying wire, on that side of the wire which is away from the web, whereby this box can direct against the web an underpressure which is controllable in the machine's cross direction. The underpressure box creates an underpressure region (34′) between the leader and the underpressure box, the region having an underpressure p1 at the detachment point, i.e. in the area where the wire is detached from said drying cylinder (14), which underpressure is sufficient to detach the leader from said drying cylinder. A protection zone is created between the underpressure region created at the leader and the run-down strip, which protection zone has a barrier (46) in order to maintain the underpressure p1 in the underpressure region.