The separator for storage battery of the present invention is a separator for storage battery mainly composed of microfibrous glass and an expanded microcapsule which has been kept in shape with its shell rendered water-permeable by expansion is incorporated in the aforesaid microfibrous glass so that an electrolyte can be retained in the gap between the glass fibers and in the expanded microcapsule to provide a high electrolyte retention and allow the aforesaid expanded microcapsule to act as a cushioning material, whereby the separator is provided with an enhanced restoring force under pressure and is thus kept the adhesion to the electrode over an extended period of time, making it possible to attain the enhancement of the storage battery capacity and the prolongation of its life and apply only a low pressure to incorporate the electrode group in the battery case during assembly of storage battery.