A belt sander comprising: a main casing; a belt unit including a frame arm that is rotatably connected to said main casing, and an abrasive belt that is mounted to said frame arm and that is adapted to be driven by electric power; a positioning unit including a positioning member that is movably mounted to said main casing, and an engaging member that is co-movably connected to said positioning member, said positioning member being movable relative to said main casing between a first engaging position, where said engaging member engages said frame arm such that said frame arm is prevented from rotating, and a releasing position, where said engaging member is disengaged from said frame arm such that said frame arm is allowed to rotate; and a sensor unit including a first sensor that is mounted to said main casing, said belt sander being characterized by: a second sensor that is connected to one of said positioning member and said engaging member, and that is co-movable with said one of said positioning member and said engaging member relative to said first sensor, said first and second sensors (61, 62) being convertible between a first sensor state and a second sensor state; wherein, when said positioning member is at the releasing position, said first and second sensors (61, 62) are in the first sensor state, and said second sensor generates a first signal for ceasing the electric power; and wherein, when said positioning member is at the first engaging position, said first and second sensors (61, 62) are in the second sensor state, and said second sensor generates a second signal for conducting the electric power. a main casing; a belt unit including a frame arm that is rotatably connected to said main casing, and an abrasive belt that is mounted to said frame arm and that is adapted to be driven by electric power; a positioning unit including a positioning member that is movably mounted to said main casing, and an engaging member that is co-movably connected to said positioning member, said positioning member being movable relative to said main casing between a first engaging position, where said engaging member engages said frame arm such that said frame arm is prevented from rotating, and a releasing position, where said engaging member is disengaged from said frame arm such that said frame arm is allowed to rotate; and a sensor unit including a first sensor that is mounted to said main casing, said belt sander being characterized by: a second sensor that is connected to one of said positioning member and said engaging member, and that is co-movable with said one of said positioning member and said engaging member relative to said first sensor, said first and second sensors (61, 62) being convertible between a first sensor state and a second sensor state; wherein, when said positioning member is at the releasing position, said first and second sensors (61, 62) are in the first sensor state, and said second sensor generates a first signal for ceasing the electric power; and wherein, when said positioning member is at the first engaging position, said first and second sensors (61, 62) are in the second sensor state, and said second sensor generates a second signal for conducting the electric power. a positioning unit including a positioning member that is movably mounted to said main casing, and an engaging member that is co-movably connected to said positioning member, said positioning member being movable relative to said main casing between a first engaging position, where said engaging member engages said frame arm such that said frame arm is prevented from rotating, and a releasing position, where said engaging member is disengaged from said frame arm such that said frame arm is allowed to rotate; and a positioning member that is movably mounted to said main casing, and an engaging member that is co-movably connected to said positioning member, said positioning member being movable relative to said main casing between a first engaging position, where said engaging member engages said frame arm such that said frame arm is prevented from rotating, and a releasing position, where said engaging member is disengaged from said frame arm such that said frame arm is allowed to rotate; and a sensor unit including a first sensor that is mounted to said main casing, said belt sander being characterized by: a second sensor that is connected to one of said positioning member and said engaging member, and that is co-movable with said one of said positioning member and said engaging member relative to said first sensor, said first and second sensors (61, 62) being convertible between a first sensor state and a second sensor state; a first sensor that is mounted to said main casing, said belt sander being characterized by: a second sensor that is connected to one of said positioning member and said engaging member, and that is co-movable with said one of said positioning member and said engaging member relative to said first sensor, said first and second sensors (61, 62) being convertible between a first sensor state and a second sensor state; wherein, when said positioning member is at the releasing position, said first and second sensors (61, 62) are in the first sensor state, and said second sensor generates a first signal for ceasing the electric power; and wherein, when said positioning member is at the first engaging position, said first and second sensors (61, 62) are in the second sensor state, and said second sensor generates a second signal for conducting the electric power.