This invention provides a web expander comprising means for carrying a web 2 having longitudinal slits, means 5 for expanding the width of the web, and opening control means 6 for uniformly forming size and form of openings of the web expanded by said means for expanding. The means 5 for expanding the width consists of a flat expander 15 and two pairs of cloth guider 21, and opening control means 6 comprises two expander rolls 31 as a rotatable ribbed roll, the ribbed roll being adapted to be applied to one side of the traveling net-like web to rotate in the direction in which the net-like web 2 is reduced in width.