The energy required to remove an electron from a gaseous atom and converted into a electropositive ion, is called Ionisation potential.
On moving down a group atomic number and atomic size both are increases due to the addition of extra shells. Hence the force of attraction between the nucleus and the valence electron is weaker. So it becomes easy to remove an electron from an atom, i.e. it requires lesser amount of energy. Hence ionization potential decreases down the group.