  T'-RE2CuO4 (RE: rare-earth element), after appropriate "reduction", has
fairly high conductivity and also exhibits clear Fermi edge in photoemission
spectroscopy. To clarify the origin of conductivity in the T' mother compounds,
we evaluated the unscreened charge-transfer gap for T'-RE2CuO4 and T-La2CuO4.
The value of charge-transfer gap for T'-compounds almost linearly decreases
with increasing the ionic radius of RE from 12.24 eV for T'-Tm2CuO4 to 9.90 eV
for T'-La2CuO4. The results qualitatively explain metallic conductivity in
T'-RE2CuO4 for large RE.
