PURPOSE: To prevent a printed substrate from warping or cracking and to perform a waterproofing having excellent mass productivity by forming a long recess in which the protrusion of a retaining die is submerged in a filler in a direction perpendicular to the longitudinal direction of a case on the filler.

CONSTITUTION: A waterproofing method for a printed substrate pours a predetermined quantity of uncured liquidlike filler 2 in a case 1 made of a synthetic resin molding. Then, a printed substrate 4 on which an electronic component 3 is mounted is so disposed as to submerge it in the filler 2. Thereafter, a retaining die 5 formed with a long protrusion 5a in a direction perpendicular to the longitudinal direction of a case 1 is so disposed as to submerge the protrusion 5a in the filler 2, and thermally cured in this state. After the filler 2a is completely cured, the die 5 is removed, and a long recess 2b is formed on the part in which the protrusion 5a of the die 5 is submerged in the filler 2a. Since the temperature of a whole printed substrate device is reduced and the shrinkage of the case 1 is absorbed by the recess 1b, the 'warpage' of the printed substrate device and the crack of the case 1 can be eliminated.