PURPOSE: To provide a sialidase inhibitor containing at least one kind of compound selected from aloe emodin and sennidin and having high activity.

CONSTITUTION: The objective sialidase inhibitor expected to be effective as an antiviral agent, anti-inflammatory agent, antibacterial agent and contraceptive is produced by using at least one kind of compound selected from aloe emodin and sennidin (composed of two kinds of isomers, i.e. sennidin A and sennidin B) expressed by formula I and formula II, as active component. The compound is an anthraquinone compound separated from a herb drug such as aloe DAIO (rhizome of Rheum palmatum) and senna by extraction. Sennoside which is a glycoside of sennidin is produced in larger amount than sennidin by the extraction and separation treatment of the above herb drug and the sennoside can be converted to sennidin by conventional hydrolysis.