Provided are: a maturation suppressor for dendritic cells, said maturation suppressor inhibiting the binding of TARM1 protein to collagen protein to thereby suppress the maturation of dendritic cells; a maturation suppression method for dendritic cells, said method comprising using the maturation suppressor; a pharmaceutical composition containing the maturation suppressor as an active ingredient; a method for screening a maturation suppressor for dendritic cells; and a method for screening a therapeutic component for a disease mediated by dendritic cells.