The present invention relates to oxygen-containing heterocyclic compounds represented by the following formula (I):
1
wherein m indicates an integer of 0 to 4;
R
1
, R
2
, R
3
and R
4
are the same or different, and each represents a hydrogen atom, substituted or unsubstituted lower alkyl, substituted or unsubstituted cycloalkyl, etc.; R
5
represents hydroxy or substituted or unsubstituted lower alkoxy; R
6
represents a hydrogen atom or halogen; and Y represents the following formula (i) or formula (ii):
2
or pharmaceutically acceptable salts thereof.