  Entanglement is a fundamental property in quantum mechanics that systems
share inseparable quantum correlation regardless of their mutual distances.
Owing to the fundamental significance and versatile applications, the
generation of quantum entanglement between {\it macroscopic} systems has been a
focus of current research. Here we report on the deterministic generation and
tomography of the macroscopically entangled Bell state in a hybrid quantum
system containing a millimeter-sized spin system ($\sim 1\times10^{19}$ atoms)
and a micrometer-sized superconducting qubit. The deterministic generation is
realized by coupling the macroscopic spin system and the qubit via a microwave
cavity. Also, we develop a joint tomography approach to confirming the
deterministic generation of the Bell state, which gives a generation fidelity
of $0.90\pm0.01$. Our work makes the macroscopic spin system the {\it largest}
system (in the sense of atom number) capable of generating the maximally
entangled quantum state.
