PURPOSE: To enable the masking with a single sticking operation easily by a method wherein a masking seal whose shape is similar to that of the arrangement of lead pins of an after mounting component is used to mask pin holes of the after mounting component together.

CONSTITUTION: A masking seal 18, whose shape is equal to that of the arrangement of lead pins 14 of a component 12 inserted into pin holes 16 which are to be masked, is previously formed by frame-cutting a masking tape. The pin holes 16, into which the pins 14 of the component 12 are to be inserted, are covered with a single masking seal 18. Therefore, an operator separates the masking seal 18 from the masking tape and sticks it on a masked part. By these processes, parts required to be masked can be masked with a piece of masking seal without cutting a mask tape into pieces for each hole and sticking them, so that holes scattering in a wide range can be easily masked.