A laser strainmeter with a 1500 m baseline was constructed at an underground site in Kamioka, Gifu Prefecture, Japan. The interferometer using a high frequency-stabilized laser measures the change in distance between two retroreflectors in the tunnel of the KAGRA gravitational-wave telescope. Since operations began in August 2016, ground motions with various timescales have been detected. The tidal waveform agrees with the theoretical waveform after topographic corrections. The strain spectrum indicates the lowest background noise of less than 10−12 in the mHz band. The strainmeter provides a new method for observing low frequency ground motion on seismic, geodetic, and intermediate timescales.