the drawing and the positions of the swell portions 42 or recess portions 43 of the waves on the friction plate 4 are arranged to always produce a shift in phase therebetween as depicted in fig3 the distribution of a bearing stress can be rendered substantially even accordingly the pressing raised portions of a piston may be constructed such that as illustrated in fig8 each pressing raised portion extends over two or more of swell portions 42 or recess portions 43 of waves on the friction plate 4 needless to say it is also necessary in this construction to set the positional relationship between the pressing raised portions 31 and the swell portions 42 or recess portions 43 of the waves on the friction plate 4 such that they are shifted in phase the present invention therefore excludes such a construction that as illustrated in fig9 the pressing positions of pressing raised portions 32 of a piston 2 and the swell portions 42 or recess portions 43 of waves are registered in phase with each other concerning the number of the pressing raised portions 21 or 31 on the piston and the number of the waves one of the numbers may be an even number and the other number may be an odd number at least the pressing raised portions or the waves may be arranged unevenly on and along a.