A flexible tube used as a part of an insertion section which defines a central axis of an insertion apparatus to be inserted into a hole, the flexible tube includes: a helical tube comprising a closely-wound region including a concave-convex portion which is provided on each of a pair of edges of a strip-shaped member elongated along a longitudinal axis, and which alternately includes convex portions and concave portions on the concave-convex portion, wherein, in a state where the strip-shaped member is spirally wound, the convex portions of the concave-convex portions facing each other along the central axis are compressed to each other; and a cylindrical outer tube which covers an outer surface of the helical tube.