PURPOSE: To obtain a case well airtight with less cracking and chipping by forming a mixture of short-fiberlike filler and a thermosetting resin into a sheet to be solidified prior to molding in a specific form.

CONSTITUTION: A filler such as nylon, carbonfiber and cotton in a fiber length of 1W10mm is arranged into a resin at a rate of about 0.7W20wt%. Blending of more than one kind of fillers is possible and silica, alumina or the like can be used with the filler. Thermosetting epoxy, phenol or the like is used for the resin to which a hardening agent, a hardening promotor, a pigment, a dye and an agent for imparting noncombustible property are added. After mixed evenly, the material is run and solidified in a specified shape to mold. For example, when sealing material 11 thus formed is applied to a semiconductor element 4 placed on a substrate 3 and hardened by heat, it seals the element well airtight easily with limited cracking and chipping.