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Summary form only given. In order to obtain a high-current charged particle beam, two types of linear induction accelerators (LIA), that is, unidirectional pulse injection type and bidirectional pulse injection type accelerators have been constructed. In the former type accelerator, which is commonly used at present, a unidirectional voltage pulse is injected to accelerating cavities, in which toroidal magnetic cores surrounding a beam path are usually loaded to increase the shunt inductance. Since the magnetic energy stored in the core during the pulse is impossible to be recovered, the energy transfer efficiency of the accelerator system is relatively low. On the other hand, in the latter type, an accelerating cavity is formed from pulse forming lines. In this case, no magnetic core is needed to be loaded in the cavity. A bidirectional voltage pulse is injected and the voltage with reverse polarity is used to accelerate a beam. Therefore, it is possible to make the time integral of the pulse voltage zero. Thus the final magnetic energy stored in the cavity can be made zero at the end of the pulse. Therefore, the accelerator system attains the theoretical energy transfer efficiency of 100%. Two pulse generators which consist of three pulse forming lines of arbitrary impedances are analyzed using the Bergeron method (Kotta et al., 1984). As a result, two bidirectional pulse generators of external pulse injection type with energy transfer efficiency of 100% are obtained. We constructed one of the pulse generator. The measured voltage wave form agrees well with the computed one. However, from the experimental results, it was also confirmed that the inductance and the resistance of the output switch influences the output wave form, in particular the rise time of the output voltage.