A sheet conveyer has a casing defined with first and second conveying passages and respectively having first and second curved passages. The first and second curved passages curve in opposite directions. A flap rotatably provided to a second guide member constituting a curved outside part of the first curved passage. When the flap is at a first position, the flap blocks passage from a part of the first conveying passage to the connection position, while the second conveying passage is opened to the connection position. When the flap is at the second position, the first conveying passage is opened toward the connection position and a part of the second conveying passage is blocked with respect to the connection position. A guide surface constituting an upper surface of the first curved passage is closer to the third guide member than a guide roller when the flap is at the second position.