The vessel monitoring system has, in a trial navigation by provisionally setting a value of the speed of the own vessel arbitrarily, a display unit immediately display an Obstacle Zone by Targets (OZT) corresponding to the speed of the own vessel.The vessel monitoring system includes a calculator for calculating an Obstacle Zone by Targets, a display unit for displaying the Obstacle Zone by Targets obtained by the calculation by the calculator, and an integrated controller for processing trial navigation that integrally controls mutually cooperated processing of calculation by the calculator and display by the display unit, in a trial navigation by provisionally setting a value of the speed of the own vessel arbitrarily, so that the calculator calculates an Obstacle Zone by Targets corresponding to the arbitrarily and provisionally set value of the speed of the own vessel, and the display unit displays the result of the calculation.