The detailed structure of electron-photon showers is investigated by simulating the physical processes accurately with the Monte Carlo method. The distributions of characteristic quantities of the showers in aluminium are given for electron- and photon-initiated showers at the incident energy of 400me (me: the electron rest mass). The longitudinal transitions, energy distributions and radial distributions for the average numbers of electrons and photons in a shower are given, and the interrelations between the distributions of electrons and of photons are discussed. The spatial distributions of energy loss by shower particles are also presented. Furthermore, fluctuations in the number of electrons at a given depth and the energy loss in a specified volume are given. Some of these distributions are compared with other authors' results.