We proposed a gas sensor based on viscosity change of ionic liquid. Our sensor consists of a piezoresistive cantilever immersed in an ionic liquid droplet. When the ionic liquid absorbs gas, the viscosity of the liquid decreases. Thus, the gas concentration is estimated by measuring the viscosity of the ionic liquid. We measured the viscosity change by monitoring the vibration amplitude of the immersed piezoresistive cantilever, which was vibrating in the ionic liquid by the magnetic field. The size of fabricated sensor chip was 1.5 mm × 1.5 mm × 0.3 mm. The cantilever and the volume of the ionic liquid droplet were 300 μm × 200 μm × 0.5 μm and 0.4 ×ℓ, respectively. We demonstrated that the vibration amplitude of the cantilever increased monotonically when acetone concentration rose from 0 to 15 mol%.