  Gas Electron Multiplier (GEM) is one of the devices for gas amplification and
available as an amplification part of detectors for many experiments. A GEM
with a 50 $\mu m$ thick insulator is popular and widely used in many
experimental fields. However it is necessary to use several layers of them to
get sufficient gas gain because gas gain which is provided with only one 50
$\mu m$ thick GEM is only several times ten. On the other hand, a thick GEM
whose amplification area is expanded from 50 to 100 $\mu m$ to get higher gas
gain which can be sufficient even with a double stack configuration. But the
measurement of gas gain uniformity using a large area 100 $\mu m$ thick GEM
reported here observed sizable non-uniformity which reached more than 50\%. We
investigated gas gain uniformity which is derived from geometries of the GEM
using $\rm{garfield^{++}}$ and considered an optimal geometry for the 100 $\mu
m$ thick GEM.
