We demonstrated a very low-noise, large-area calorimeter based on a micromechanical heat-flow sensor. The input heat flow is transduced to the bimetallic deformation of the sensor, which is observed using a high-resolution fiber optic interferometer. The characteristics of 5-mm-square Si-Al bimetallic micromechanical sensors fabricated using silicon-on-insulator micromachining processes were investigated. We experimentally achieved a heat-detection noise of 12nW, which is comparable to the best performance of thermopiles. The developed sensor is free of Johnson noise, which constrains the signal-to-noise ratio. In principle, this technique can provide a sub-nanowatt resolution even in air at room temperature. (C) 2016 The Japan Society of Applied Physics