Driven magnetic reconnection in a collisionless plasma is investigated by means of a two-and-one-half dimensional particle simulation. The dynamical compression by plasma inflow forms a peaked profile of current density in the neutral sheet. When the width of current layer is compressed as thin as the ion Larmor radius, the charge separation becomes distinct abruptly at the center of the current layer due to the finite ion Larmor radius effect. The charge separation in the central current region and the subsequent spatial modification of the current profile result in excitation of collisionless driven magnetic reconnection. In the cause of collisionless driven magnetic reconnection an efficient energy conversion from field energy to particle energy is observed. (author).