Abstract: A leak check device is for a seal structure of a junction between a first component and a second component. The seal structure is configured such that a fluid passage is defined inside the first and second components connected together, a seal member is attached to the first component, and the seal member prevents leakage of a fluid through the junction. The leak check device includes a test plug configured such that (a) the test plug includes an attachment portion to which the seal member is attached, (b) the attachment portion is connected to the second component separated from the first component, (c) a closed space is defined by an inner peripheral surface of the second component, an outer peripheral surface of the seal member, and an outer peripheral surface of the attachment portion, and (d) the test plug includes an introduction portion through which a test fluid is introduced into the closed space.