Provided is an artificial nucleoside or artificial nucleotide capable of composing an artificial oligonucleotide having superior nuclease resistance. The artificial nucleoside or artificial nucleotide is a compound represented by formula (I) or a salt thereof (wherein, Bx represents a pyrimidine base or purine base, R1, R2, R3 and R4 represent hydrogen atoms, C1-6 alkyl groups or the like, Y represents NR5R6 (wherein, R5 and R6, independently of each other, represent a hydrogen atom, C1-6 alkyl group or the like, or R5 and R6, together with a nitrogen atom bound thereto, form a 3- to 11-membered nitrogen-containing non-aromatic heterocyclic group) or an optionally substituted C2-9 aromatic heterocyclic group, Z1 and Z2 represent hydrogen atoms, hydroxyl group-protecting groups, phosphorous-containing groups or the like, and n represents an integer of 1 to 3).