This invention provides a method for inspecting a joined region and joining strength of a joined object formed by a spot friction stir joining process. In this method, an ultrasonic wave is introduced into the joined object from a backing face 25 opposed to a joining tool plunging face 24 and the ultrasonic wave reflected from the joined object 20 is taken in. In this case, the joining region 21 and the joining strength is estimated by observing a reflected wave of the ultrasonic wave in the vicinity of a position, along a reference direction Z, corresponding to an interface 27 of two joining members, without using the reflected wave of the ultrasonic wave reflected from the joining tool plunging face, thereby inspecting the joined object based on the estimation result. Thus, influence of a concave/convex shape of a joining mark 29 formed in the joining tool plunging face 24 can be avoided, and the use of the ultrasonic wave for estimating the joined region 21 can achieve an inspection of joining quality without destroying the joined object 20.