The present invention provides TLR9 agonists comprising, as an active ingredient, a compound represented by formula (I):
(wherein a represents 0 or 1; n represents an integer of 0 to 2; m represents an integer of 0 to 5; X
1
and X
2
each independently represent a hydrogen atom or hydroxy; Y represents an oxygen atom or a sulfur atom; -Q
1
-represents —O— or the like; -Q
2
- represents —O— or the like; -Z- represents —O— or the like; R
1
, R
3
and R
4
each independently represent hydroxy or the like; R
2
and R
5
each independently represent a hydrogen atom, hydroxy or the like; and A represents 6-aminopurin-9-yl or the like) or a pharmaceutically acceptable salt thereof, and the like.