Otaki (the Big Waterfall) in Osori in Nishiizu Town, Shizuoka Prefecture, is a waterfall with a height of 48 meters and a width of 8 meters. It is one of the representative waterfalls in Izu Peninsula and is the biggest waterfall of over twenty waterfalls located in the upstream of the Nishina River and other rivers in the town.

The Otaki Waterfall is a literally big and dynamic waterfall with a large basin. When the river volume increases, it flows straight down the cliff with splashes of water. There is a slight terrace in the middle of the cliff, from which water flows down in a folding-fan shape. It is also called Heitagataki or Sankaitaki (the Three-story Waterfall) because it flows down in three stages. A rainbow can be seen, depending on the direction of the flow.

Visitors can enjoy seasonal changes in scenery including tender green in early summer and crimson foliage in fall.