Process for polymerizing olefins and polymerisation catalyst

Catalyst and preparation method of (A), (B) and (C) is formed: (A) containing magnesium, titanium, a halogen and polycarboxylic acid ester as a main component, and by bringing a magnesium compound, a titanium compound and a polycarboxylic acid the solid titanium catalyst obtained by contacting the ester; (B) an organoaluminum compound; (C) represented by the following formula SiR