Silicon nitride (Si3N4)-on-SiO2 attracts increasing interest in integrated photonics owing to its low propagation loss and wide transparency window, extending from ～400nm to 2350 nm. Scalable integration of active devices such as amplifiers and lasers on the Si3N4 platform will enable applications requiring optical gain and a much-needed alternative to hybrid integration, which suffers from high cost and lack of high-volume manufacturability. We demonstrate a high-gain optical amplifier in Al2O3:Er3+ monolithically integrated on the Si3N4 platform using a double photonic layer approach. The device exhibits a net Si3N4-to-Si3N4 gain of 18.1±0.9dB at 1532 nm, and a broadband gain operation over 70 nm covering wavelengths in the S-, C- and L-bands. This work shows that rare-earth-ion-doped materials and in particular, rare-earth-ion-doped Al2O3, can provide very high net amplification for the Si3N4 platform, paving the way to the development of different active devices monolithically integrated in this passive platform.