To accurately control a sealant penetration position in a stuck substrate to prevent over-penetration or under-penetration of a sealant with respect to a method and device for manufacturing a liquid crystal display panel, which seal a liquid crystal charging port in the stuck substrate where liquid crystal has been charged.

The device for manufacturing the liquid crystal display panel is constituted of a light source 2 which radiates light on an optical path passing the vicinity of a desired penetration position of a sealant 20 at the time when a stuck substrate 14 in which a liquid crystal charging port 26 is coated with the sealant 20 is placed, a pair of polarizing plates 4 and 6 which are arranged on the optical path on both sides of the placed stuck substrate 14 so that their axes of polarization may be parallel, a detector 8 which detects the intensity of transmission light transmitted through the polarizing plates 4 and 6 to output a transmission light intensity signal, a control part 12 which outputs a sealant hardening signal on the basis of the transmission light intensity signal, and a UV lamp 10 which hardened the sealant 20 on the basis of the sealant hardening signal.