A highly polymerizable N-vinylcarboxylic acid amide having an N-1,3-butadienylcarboxylic acid amide content of 30 ppm or less, a process for producing the same, and a process for producing a homopolymer of N-vinylcarboxylic acid amide or a copolymer thereof with another copolymerizable monomer using the same. Also, a highly polymerizable N-vinylcarboxylic acid amide is produced by thermal cracking or catalytic cracking of N-(1-alkoxyethyl)carboxylic acid amide or ethylidenebiscarboxylic acid amide, wherein the N-vinylcarboxylic acid amide content of the N-(1-alkoxyethyl)carboxylic acid amide or ethylenebiscarboxylic acid amide is 10 wt % or less.