There have been presented many investigations of the stress distribution of superstructure. In these investigations, it may be assumed that the H. H. Bleich's theory is most useful to estimate the longitudinal stress distribution of long deckhouse, however, shear lag effects are neglected in his theory so that this theory gives good agreement with experimental results in the case of long deckhouse except in the vicinity of the house ends, but is not appropriate to the case of short deckhouse. In this paper, to obtain the stress distribution of short deckhouse, the authors present bending theory of superstructure taking into consideration of shear lag effects, and this theoretical values are compared with experimental results obtained by bending tests of box girders, with a good agreement.