To obtain a composition which increases prostatomegaly inhibitory effect by including a dioxabicyclooctane derivative and Serenoa repens oil or the like.

This composition is obtained by including (A) a dioxabicyclo[3,3,0]octane derivative, e.g. sesamin, sesaminol, episesamin, episesaminol, sesamolin or the like, of the formula [R1-R6 are each H or a 1-3 C alkyl; R1 and R2 or R4 and R5 are respectively combined to form methylene or ethylene; (n), (m) and (l) are each 0 or 1] and (B) Serenoa repens [another name: Saw-palmetto or the like, belonging to a palm widely inhabiting the south of North America continent] oil or Serenoa repens fruit dry powder containing the oil. On the above composition, the component A is preferably included in a proportion of more than 1 wt.% based on the component B (Serenoa repens oil).