This invention discloses organogermanium compounds characterized in being expressed by the following formula: ##STR1## wherein R.sub.1, R.sub.2 and R.sub.3 respectively denote a hydrogen atom, a lower alkyl group, or a substituted or unsubstituted phenyl group, Y denotes an oxygen or sulfur atom, and Z denotes a hydroxyl group, an amino, or a lower alkoxy group, and antitumor agents comprising as a principal agent these compounds.