In structural analysis of frames by the efficient and accurate method (Idealized Structural Unit Method), members are usually assumed to be rigidly connected to each other at nodal points. However, in tubular frames with simple (unstiffened) joints, the joints may show considerable flexibility in the elastic as well as the elastic-plastic ranges. Such flexibility may cause excessive deflections and different internal force distribution in the structure. Increased deflections are of great concern since their effects may be great and important in the collapsing behavior of a structure. In this paper, the authors carried out theoretical investigations into the local rigidity and strength of tubular joints (T, Y, TY and K joints). Based on the results, they idealized the behavior and developed an effective simple model of tubular joints, which is constructed with spring, rigid bar and beam elements. This model is named an idealized “Tubular joint element”. The model exhibits joint flexibility in the elastic range as well as the elastic-plastic range and the limit strength, following the elastic fully-plastic load-displacement relationships which are assumed based on the results obtained either by theoretical analysis or experiment. Under several typical loading conditions, the behavior of the model was examined, being compared with the results of analyses by the finite element method (the plastic node method). The result confirmed the accuracy and assured the validity of the model.