A medicament for enhancing an effect of a cancer therapy based on a mode of action of DNA injury, which comprises as an active ingredient a compound represented by the following general formula (I) or a salt thereof:
wherein one of R
1
and R
2
represents hydrogen atom and the other represents the formula —X-A wherein A represents hydrogen atom or an acyl group, X represents oxgen atom or NH; one of R
3
and R
4
represents hydrogen atom and the other represents the following formula:
wherein Y represents a sulfonyl group or a carbonyl group, R
5
represents a cyclic group, Z represents a single bond or a C
1
to C
4
alkylene group, R
6
represents hydrogen atom or a C
1
to C
6
alkyl group.