Interest in complex fluids which show the shear thickening behavior is rapidly increasing. To understand the mechanism of shear thickening of such a fluid, a network model is proposed. The rates of creation and loss of the crosslinks are given as an exponential function of effective strain, which is defined as a ratio of the normal stress difference and the shear stress. A sufficient condition for shear thickening is suggested, which comes from the different speeds in the process of creation and loss. The theory is compared with the shear flow properties of a 2% aqueous poly(vinyl alcohol) solution containing 1% sodium borate. It is argued that the shear thickening of this solution arises from the fact that the rate of loss decreases more quickly than that of creation. Some characteristics of this complex fluid are also investigated in terms of the network structure.