Research teams at JAEA, the University of Tokyo, and the Institute for Materials Structure Science, High Energy Accelerator Research Organization (KEK) have used the total reflection high-energy positron diffraction (TRHEPD) method to determine the atomic arrangement of germanium, a germanium version of the monoatomic layered material graphene. Unlike graphene, germanene does not exist in nature, but like graphene. It has attracted attention as a new material for nanotechnology, being studied intensively around the world ever since its synthesis on metal substrates was reported recently. Though several proposals have been made for the atomic arrangement of germanene, no experimental structure determination has been reported yet. In this research the structure of germanium on an aluminum substrate was determined using the TRHEPD method. As a result, it became clear that the symmetry of the atomic arrangement was broken, contrary to wide expectations. With the understanding of the basic atomic arrangement now made, it is expected that device development using germanene will progress toward the realization of energy-saving, high-speed, and compact electronic devices.