To obtain a polyisocyanurate foam of a hydrophilic fine open cell suitable for an artificial callivating medium for a nursery or a supporter for a cut flower.

In producing a polyisocyanurate foam comprising a polyisocyanate compound, a hydroxy compound containing ≥50 wt.% polyoxyethylene content, a blowing agent, a catalyst and a polydimethylsiloxane- polyoxyalkylene copolymer as essential components, both two kinds of a polydimethylsiloxane-polyoxyalkylene copolymer having foam stabilizing properties and a polydimethylsiloxane-polyoxyalkylene copolymer having a specific structure and no foam stabilizing properties are used.