PURPOSE: To realize the holding of an airtight state and a long-time inspection and to increase the detecting capacity, by holding the airtightness of both a container and a basic material by means of a sealing material and at the same time securing a vacuum state at the inside of the container after leading soapy water into the container so that the soapy water has a thickness.

CONSTITUTION: The soapy water 5 which is previously put into vacuum with its inside air extracted is poured into a container 3 via a pipe 4 until a basic material 1 and a welded part 2 which are enclosed by the container 3 sink under the water 5. Then the pipe 4 is connected to a vacuum pump to evacuate the inside of the container 3, and the container 3 is left as it is for several minutes. Then the inside of the container 3 is observed through a window to know the presence or absence of bubbles. In such a way the water 5 functions as a liquid seal to ensure the sufficient airtightness between the container 3 and the material 1. Furthermore the water 5 has a thickness and is not easily dried to realize a long-time inspection. As a result, the inspecting capacity is increased for the leakage.