To restrain a blade made of an aluminum extruded material of a ventilation louver installed outdoors as a fence, from swinging hard on the front and back faces to make flapping sound and to make whistling sound in about 40 m/s or more wind speed in a wind tunnel test.

The blade 2 is formed by arranging blindfold protrusions 23 at the upper and lower parts of an inclined base part 21 through gentle inclined parts 22, and arranging tapping holes 24 in the blindfold protrusions 23, while installing a wind receiver fin 25 facing the back face side, on the tapping hole 24 of the lower cover protrusion 23. Lift on the front face side and back face side of the blade 2 is made small on the front face side and large on the back face side to suppress swinging of the blade 2. A curved part 26 facing the front face side is arranged above the upper blindfold protrusion 23 of the blade 2 to suppress whistling sound.