Abstract: The invention relates to a 3'-substituted nucleoside derivative represented by the following general formula (1): ##STR1## wherein B means a nucleic acid base which may have a substituent, Z represents a lower alkynyl or lower alkenyl group which may be substituted by a group represented by the formula: ##STR2## in which R.sup.a, R.sup.b and R.sup.c are individually a lower alkyl group or a phenyl group, or an oxiranyl group which may have at least one lower alkyl group, R.sup.1 and R.sup.2 individually represent H or an ester-forming residue capable of easily leaving in a living body, and R.sup.3 is H, a mono- or polyphosphoric acid residue, or an ester-forming residue capable of easily leaving in a living body, with the proviso that the sugar moiety is ribose, or a pharmaceutically acceptable salt thereof. The 3'-substituted nucleoside derivative according to the invention has an excellent antitumor activity and is hence useful for treatment for and prevention of cancers.