To enable the easy adjustment of the height of the upper end of a shoulder belt of webbing utilizing a shoulder guard provided to a strut provided on both right and left sides of a seat. The shaft includes a flange portion formed in the end joined to the ceramic substrate and a first cylindrical portion connecting to the flange portion, and a ratio D3D1 of an inner diameter D3 of a joint area in the flange portion and an outer diameter D1 of the first cylindrical portion is not less than 92%. The sprinkler can include a diverter valve that can intermittently block or permit the flow of water to the high flow rate port andor to the low flow rate port as the nozzle rotates.