Abstract: A crystal oscillator unit which serves as a flat packaged piezoelectric device includes a base member, a quartz piece, and a cap member sealed air-tight to the base member to enclose the quartz piece in cooperation with the base member. Each of the base and the cap members is made of a glass-ceramic composite material in which 30 to 70% by weight of forsterite is dispersed in glass and which has a thermal expansion coefficient between 100.times.10.sup.-7 /.degree.C. and 150.times.10.sup.-7 /.degree.C. The glass consists, in percent by weight, essentially of 50 to 70% of SiO.sub.2, 3 to 15% of Al.sub.2 O.sub.3, 0 to 10% of ZnO, 0 to 8% of B.sub.2 O.sub.3, 7 to 25% of at least one member of CaO, SrO, and BaO, and 13 to 27% of at least one member of Na.sub.2 O, K.sub.2 O, and Li.sub.2 O. The glass and the forsterite are pulverized to an average particle diameter of 1 to 3 microns, mixed together with addition of fine powder of an average particle diameter of 0.