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We report the experimental results of in situ detection of terahertz radiation from large stacks with a few hundreds of intrinsic Josephson junctions (IJJs) using a high sensitivity detector composed of a small IJJ mesa. The dimensions of the fabricated IJJ oscillators are 290 μm in length, 50, 70, and 90 μm in width and between 255 and 600 nm in height. The current-voltage characteristics of the oscillators are strongly influenced by self-heating and show negative resistance. We have successfully observed terahertz radiation from stacks with 170-400 IJJs. We find that the strong emission occurs when the oscillator is biased in the negative resistance region on the outermost quasi-particle branch. The characteristic velocity estimated from the emission voltage is consistent with that of the in-phase mode. This implies that the observed emission is related to the in-phase cavity resonance. Furthermore, we find that for a smaller number of junctions the emission corresponds to higher harmonics of the cavity resonance.