A phase space structure of the electron distribution function is investigated by the gyrokinetic theory and numerical simulations to investigate a possible mechanism of the excitation of the beam instability which induces collisionless magnetic reconnection in a strong guide field. It is shown that the perturbed electron distribution function develops in proportion to the shifted Maxwellian distribution as the reconnection electric field accelerates electrons along the guide field at the X-point, with parity symmetry around the z-axis. The accelerated electrons are expected to excite the kinetic Alfvén waves (KAWs) when the beam velocity exceeds the Alfvén speed. The obtained results suggest a possible scenario for anomalous resistivity generation in the case with the strong guide field where the beam electrons accelerated at the X-point lose their parallel momentum through interactions with the self-excited KAWs.