Provided is a reagent, which prolongs a blood coagulation time sufficiently and enhances an optical change, thereby enabling a correct and high-sensitive blood coagulability test. The present invention provides a blood coagulation time prolonging agent, including, as an active ingredient, a guanidine compound represented by the following formula (1) or an acid addition salt thereof (in the formula, R 1 represents a hydrogen atom, an amino group, or an alkyl group which may have a substituent).
