The object of the present invention is to provide a formulation with the effect of effectively suppressing or inhibiting amyloid fibril formation by the dissolution, elimination (discharge), etc. of amyloid fibril formation in vivo.If an agent for suppressing or inhibiting an amyloid fibril formation comprising tranilast or a pharmacologically acceptable salt thereof as an active ingredient is administered by a method such as oral administration, amyloid fibril formation can be effectively suppressed or inhibited in vivo as a result of effects such as amyloid fibril dissolution or elimination (discharge). Therefore, it is possible to prevent or treat amyloid plaques, in which amyloid fibrils formed by the aggregation of amyloid protein have been deposited, and to prevent or treat diseases arising from amyloid fibril deposition, that is, diseases arising from the deposited amyloid fibrils themselves and diseases that cause dysfunction of organs or tissues as a result of amyloid fibril deposition.
