To facilitate handling at transportation or the like by easily, safely and inexpensively reducing flowability of mud water or mud of a high water content ratio to an appropriate flowability in a short time.

Any one or both of a predetermined amount of polyglutamic acid and a polyglutamic acid crosslinked substance is mixed into the mud of a high water content ratio as a flowability suppressing agent to reduce the flowability of the mud of a high water content ratio.