In this paper, we propose a novel system called Downhill, which collects data for slope failure detection. Our system builds a Bluetooth Low Energy-based multi-hop communication network using smartphones. As a low-cost alternative to pile sensors, the system uses smartphones as sensor terminals to collect data. The smartphone sensors are placed in mountainous areas, which are prone to slope failures, and establish a MANET using their built-in BLE interface. If a smartphone detects ground movement, its collected acceleration data and GPS location data are sent to a server via the BLE network. These functions are executed by an application running on the smartphones. Hence, in the proposed system, it is possible to use the smartphone as the sensor terminal as long as one that has sufficient performance for use as the sensor terminal is prepared. To show the effectiveness of the system, we performed multi-hop communication experiments using Downhill in mountains that are at risk of landslides. Results show that Downhill is able to use effectively in the warning area of slope failure.