PURPOSE: To make waterproof sealing of a joint of wall panels of a bathroom, which is assembled through joining of the wall panels, easily, quickly, and with less labor in the site works without requiring skill.

CONSTITUTION: Unhardened silicon sealant 4 is encapsulated in a polyethylene tube 3 which is perforated 31 longitudinally to admit easy opening, and this tube with silicon sealant 4 is fitted in a groove 131 in a longitudinal member 12 of a wall panel 1. The wall panel 1 is butted to another 2. and a ridge 231 on a longitudinal frame member 23 is fitted in the groove 131, and thereby the wall panels 1, 2 are joined together. The polyethylene tube 3 is compressed between the groove 131 and ridge 231 to cause breakage of the perforation 31, and the sealant 4 is extruded to the joint 5 to fill it.