A medical instrument (1, 1', 2, 3, 4, 5, 5', 6) comprising: a medical device (10a, 10a', 10a", 10b, 10e'); a sheet type attaching member (20, 20') having an attachment surface (22, 22') that is attachable to a subject; and a fixing tool (100a-100f, 100a', 100e') configured to fix the medical device (10a, 10a', 10a", 10b, 10e') to the attaching member (20, 20'); wherein the fixing tool (100a-100f, 100a', 100e') comprises: a first engaging member (200a-200d, 200a', 200e, 200e', 200f') located on the medical device (10a, 10a', 10a", 10b, 10e'); and a second engaging member (300a, 300a', 300d, 300e, 300f) located on a side of the attaching member (20, 20') opposite the attachment surface (22, 22'), the second engaging member (300a, 300a', 300d, 300e, 300f) being engageable with the first engaging member (200a-200d, 200a', 200e, 200e', 200f') by rotation of the first engaging member (200a-200d, 200a', 200e, 200e', 200f') wherein the first (200a-200d, 200a', 200e, 200e', 200f') and second (300a, 300a', 300d, 300e, 300f) engaging members come in contact with each other to constitute a rotation axis of rotation, and one of the first (200a-200d, 200a', 200e, 200e', 200f') and second (300a, 300a', 300d, 300e, 300f) engaging members comprises an engagement protrusion part (220, 220a-220d, 220a'-220d', 241, 262) extending in a diameter direction around the rotation axis, and the other comprises an engagement receiving part (320, 320', 320a-320d, 320a'-320d') that is engageable with the engagement protrusion part (220, 220a-220d, 220a'-220d', 241, 262), the second engaging member (300a, 300a', 300d, 300e, 300f) comprises a support part located on the rotation axis and configured to support the first engaging member (200a-200d, 200a', 200e, 200e', 200f'); and the engagement receiving part (320, 320', 320a-320d, 320a'-320d') is spaced apart from the support part, characterized by, the second engaging member being engageable with the first engaging member by rotation of the first engaging member along a parallel direction being a direction along a plane parallel to a skin surface of the subject, and wherein an outer peripheral contour of the medical device is positioned outside an outer peripheral contour of the attaching member, when the medical instrument in the fixed state is viewed from above. a medical device (10a, 10a', 10a", 10b, 10e'); a sheet type attaching member (20, 20') having an attachment surface (22, 22') that is attachable to a subject; and a fixing tool (100a-100f, 100a', 100e') configured to fix the medical device (10a, 10a', 10a", 10b, 10e') to the attaching member (20, 20'); wherein the fixing tool (100a-100f, 100a', 100e') comprises: a first engaging member (200a-200d, 200a', 200e, 200e', 200f') located on the medical device (10a, 10a', 10a", 10b, 10e'); and a second engaging member (300a, 300a', 300d, 300e, 300f) located on a side of the attaching member (20, 20') opposite the attachment surface (22, 22'), the second engaging member (300a, 300a', 300d, 300e, 300f) being engageable with the first engaging member (200a-200d, 200a', 200e, 200e', 200f') by rotation of the first engaging member (200a-200d, 200a', 200e, 200e', 200f') a first engaging member (200a-200d, 200a', 200e, 200e', 200f') located on the medical device (10a, 10a', 10a", 10b, 10e'); and a second engaging member (300a, 300a', 300d, 300e, 300f) located on a side of the attaching member (20, 20') opposite the attachment surface (22, 22'), the second engaging member (300a, 300a', 300d, 300e, 300f) being engageable with the first engaging member (200a-200d, 200a', 200e, 200e', 200f') by rotation of the first engaging member (200a-200d, 200a', 200e, 200e', 200f') wherein the first (200a-200d, 200a', 200e, 200e', 200f') and second (300a, 300a', 300d, 300e, 300f) engaging members come in contact with each other to constitute a rotation axis of rotation, and one of the first (200a-200d, 200a', 200e, 200e', 200f') and second (300a, 300a', 300d, 300e, 300f) engaging members comprises an engagement protrusion part (220, 220a-220d, 220a'-220d', 241, 262) extending in a diameter direction around the rotation axis, and the other comprises an engagement receiving part (320, 320', 320a-320d, 320a'-320d') that is engageable with the engagement protrusion part (220, 220a-220d, 220a'-220d', 241, 262), the second engaging member (300a, 300a', 300d, 300e, 300f) comprises a support part located on the rotation axis and configured to support the first engaging member (200a-200d, 200a', 200e, 200e', 200f'); and the engagement receiving part (320, 320', 320a-320d, 320a'-320d') is spaced apart from the support part, characterized by, the second engaging member being engageable with the first engaging member by rotation of the first engaging member along a parallel direction being a direction along a plane parallel to a skin surface of the subject, and wherein an outer peripheral contour of the medical device is positioned outside an outer peripheral contour of the attaching member, when the medical instrument in the fixed state is viewed from above.