Boundwater in the passive film on Type 304 stainless steel was characterized using a thermal desorption gas analyzer. The passive film was produced electrochemically in a H2SO4 solution and was modified electrochemically in an NaCl solution. In case of producing the passive film in the H2SO4 solution, total amount of the boundwater was relatively large in the initial stage, and then decreased as the passivation time elapsed. In addition, it was found that the boundwater was classified into three types of binding states in the passive film. When the passive film was modified in the NaCl solution, the boundwater, especially of the second type, increased.