During an operation of a mobile robot 1, observed values of a plurality of reference parameters including at least one of contact reaction forces) of one or more movable links 3, 4 and a contact reaction force function value expressed as a function value of contact reaction force(s) of one or more movable links 3, 4 are acquired based on outputs from force detectors 31 mounted on the respective movable links 3, 4 of the mobile robot 1, and the observed values of the reference parameters are used to detect presence or absence of occurrence of abnormal contact of the mobile robot 1 by a contact detecting model Ai.