An ultrasonic testing method comprising a step of performing ultrasonic testing by detachably installing a vibration insulating member, which has an opening part whose width is narrower than a width of a transducer surface of an ultrasonic array probe body having transducers linearly arranged, and which absorbs vibrations of the transducer surface, onto the transducer surface so that a part of the transducer surface in the width direction of the transducer surface is exposed through the opening part and a non-exposed part of the transducer surface is in contact with the vibration insulating member, and by radiating ultrasonic waves from the transducer surface onto a surface of a test object, wherein the width dimension of the opening part is determined in advance according to a distance from a surface of the test object to a target flaw positioned near the surface of the test object, determining of the width dimension of the opening part comprises: providing a specimen with a plurality of artificial flaws each having a different distance from a specimen surface; preparing a plurality of vibration insulating members each having a different opening width dimension; performing ultrasonic testing by changing the plurality of vibration insulating members each having a different opening width dimension for individual artificial flaws; examining a range of distance from the specimen surface in which a surface echo appears, and examining an intensity of flaw echo of the respective artificial flaw, wherein the range of distance from the specimen surface in which surface echo appears and the data on the intensity of flaw echo of artificial flaw, for each distance from the specimen surface to the artificial flaw and for each opening width dimension of the vibration insulating member, are gotten together; and determining the opening width dimension at which a flaw is easily detected based on the range of distance from the specimen surface in which the surface echo appears, and on the intensity of the flaw echo. detachably installing a vibration insulating member, which has an opening part whose width is narrower than a width of a transducer surface of an ultrasonic array probe body having transducers linearly arranged, and which absorbs vibrations of the transducer surface, onto the transducer surface so that a part of the transducer surface in the width direction of the transducer surface is exposed through the opening part and a non-exposed part of the transducer surface is in contact with the vibration insulating member, and by radiating ultrasonic waves from the transducer surface onto a surface of a test object, wherein the width dimension of the opening part is determined in advance according to a distance from a surface of the test object to a target flaw positioned near the surface of the test object, determining of the width dimension of the opening part comprises: providing a specimen with a plurality of artificial flaws each having a different distance from a specimen surface; preparing a plurality of vibration insulating members each having a different opening width dimension; performing ultrasonic testing by changing the plurality of vibration insulating members each having a different opening width dimension for individual artificial flaws; examining a range of distance from the specimen surface in which a surface echo appears, and examining an intensity of flaw echo of the respective artificial flaw, wherein the range of distance from the specimen surface in which surface echo appears and the data on the intensity of flaw echo of artificial flaw, for each distance from the specimen surface to the artificial flaw and for each opening width dimension of the vibration insulating member, are gotten together; and determining the opening width dimension at which a flaw is easily detected based on the range of distance from the specimen surface in which the surface echo appears, and on the intensity of the flaw echo. providing a specimen with a plurality of artificial flaws each having a different distance from a specimen surface; preparing a plurality of vibration insulating members each having a different opening width dimension; performing ultrasonic testing by changing the plurality of vibration insulating members each having a different opening width dimension for individual artificial flaws; examining a range of distance from the specimen surface in which a surface echo appears, and examining an intensity of flaw echo of the respective artificial flaw, wherein the range of distance from the specimen surface in which surface echo appears and the data on the intensity of flaw echo of artificial flaw, for each distance from the specimen surface to the artificial flaw and for each opening width dimension of the vibration insulating member, are gotten together; and determining the opening width dimension at which a flaw is easily detected based on the range of distance from the specimen surface in which the surface echo appears, and on the intensity of the flaw echo.