Abstract: A fracture toughness testing machine of the invention makes it possible to evaluate fracture toughness of a specimen in pure mode such that the effect of thermal residual stresses is removed, when the stresses are present in the specimen obtained by bonding dissimilar materials. The testing machine includes: testing-load applying means for applying a predetermined testing load to the specimen, in which the stresses are present; and cancelling-load applying means for applying a cancelling load to the specimen to cancel the stresses therein. The cancelling-load applying means includes: a pressing-force applying portion that applies a pressing force to the specimen as the canceling load; and a pressing-force determining portion that determines magnitude of the force. The pressing-force determining portion calculates the magnitude of the force using pre-stored equations so that an energy release rate related to in-plane shear mode crack deformation becomes zero.