A polymer having a polymerizable group and an alkyleneoxy groups on side chains thereof, and a polymerizable composition containing the polymer. The polymerizable composition preferably contains a polymerizable compound and a polymerization initiator. Also provided is a planographic printing plate precursor having a polymerizable layer on a hydrophilic support, the polymerizable layer containing a polymer having a polymerizable on a side chain thereof. The planographic printing plate precursor can form an image without being subjected to an alkali development. An undercoat layer containing a specific copolymer may be provided between the support and the photopolymerizable layer.