A base plate for a power module includes: a metal plate, a ceramic base plate joined to the metal plate, and a release agent which includes boron provided in a joint surface between the metal plate and the ceramic base plate. A remaining amount of the release agent is less than 5, as an amount of boron measured by fluorescence X-ray analysis, where the amount of boron is defined as a value obtained by an expression: (a peak height of B-K.alpha./a peak height of X-K.alpha.) x 100000 and a crystal grain straining region in the joint surface is equal to or less than 40%, or an amount of crystal grain straining in the joint surface is equal to or less than 0.03%.