PURPOSE: To prevent radially irregular wears of seals without loss of sealability by forming a plurality of grooves having different lengths and extending radially form the rotating center on the end face of a core with which a crossbar seal is brought into slidable contact.

CONSTITUTION: A plurality of grooves 20A, 21A, 22A having different lengths, extending radially from a center, are formed on both ends 1B, 1C of a core 1. The meeting times of the core 1 with seals 7A, 20A-22A upon one revolution of the core 1 are increased toward the center. A seal wearing speed is proportional to a contact surface pressure, and the higher the contact surface pressure increases, the faster the wearing speed increases. The speed is accelerated by the edge effect of the groove to be effected when the seal meets with the groove. As results, the radial wearing speed can be made uniform by suitably setting the number, the length of the grooves, etc. Thus, an irregular wear can be prevented without loss of sealability.