Mitomycin compound having potent antitumour activity having the formula: ##STR1## wherein ##STR2## (wherein Ra, Rb and Rc each independently represent lower alkyl, cycloalkyl having 3-7 carbon atoms or phenyl; or Ra and Rb are bonded together to form a poly-methylene group having 2-5 carbon atoms);one of R.sub.1 and R.sub.2 represents carbamoyloxymethyl and the other represents hydrogen; or R.sub.1 and R.sub.2 may together form an exocyclic methylene group;and Y and Z each independently represents hydrogen or methyl.