  When one constructs specific models with the fundamental scale as low as the
TeV scale, there arise many difficulties. In this paper we examine the hybrid
inflation due to bulk scalar fields, which has been proposed to solve the
problem of fine-tuning in producing density perturbations. We find that the
Kaluza-Klein modes play significant roles, which enhance the speed of the phase
transition and alter the reheating process. We also argue that a lower bound
must be put to the fundamental scale, in order to construct the successful
hybrid inflation due to the bulk scalar fields.
