To obtain a design data for box beams used as headers in light-frame timber construction, $2{ imes}6;(38{ imes}140mm),;2{ imes}8;(38{ imes}184mm),;2{ imes}10;(38{ imes}235mm);and;2{ imes}12;(38{ imes}286mm)$ members were built as box beam specimens for bending tests. The allowable bending stresses for box beams were obtained through bending tests of these specimens, and span tables were calculated for various loading conditions based on the allowable bending stresses obtained. The allowable bending stresses were determined as the bending stresses at 10mm deflection of specimens from the results of bending tests of box beam specimens. Span tables for box beams were obtained assuming five loading conditions for headers used in exterior walls and two loading conditions for headers used in interior walls. Among these 7 loading conditions, 5 loading conditions applied to headers in exterior walls included the dead loads, the live loads and the snow loads and 2 loading conditions applied to headers in interior walls included the dead loads and the live loads.