A dental handpiece of a center-injection type, comprising: a bur sleeve (8, 9) having an axial through hole capable of receiving and detachably holding therein a cutting bur having an axial through hole, upper and lower bearings (10, 11) for rotatably supporting said bur sleeve (8, 9), a head casing accommodating said bur sleeve (8, 9) and upper and lower bearings (10, 11), and comprising a head housing, a cap screwed into the top of the head housing via threads, and a push button arranged in the cap, an injection nozzle communicating with a cooling water feed source at one end, and capable of being inserted into the axial through hole of the cutting bur from its proximal end for injecting cooling water through the other end, characterized in that said dental handpiece further comprises: an upper seal support attached inside the cap, a lower seal support attached to the head casing, an upper seal means in the form of an annular member having an inner and an outer periphery, and arranged around the bur sleeve (8, 9) with said inner periphery press-contacted onto the outer surface of said bur sleeve (8, 9) and with said outer periphery supported on the upper seal support, so as to seal a space between the upper region of the bur sleeve (8, 9) and the upper bearing, and a lower seal means in the form of an annular member having an inner and an outer periphery, and arranged around the bur sleeve (8, 9) with said inner periphery press-contacted onto the outer surface of said bur sleeve (8, 9) and with said outer periphery supported by the lower seal support and the lower end of the head housing, so as to seal a space between the lower region of the bur sleeve (8, 9) and the lower bearing. a bur sleeve (8, 9) having an axial through hole capable of receiving and detachably holding therein a cutting bur having an axial through hole, upper and lower bearings (10, 11) for rotatably supporting said bur sleeve (8, 9), a head casing accommodating said bur sleeve (8, 9) and upper and lower bearings (10, 11), and comprising a head housing, a cap screwed into the top of the head housing via threads, and a push button arranged in the cap, an injection nozzle communicating with a cooling water feed source at one end, and capable of being inserted into the axial through hole of the cutting bur from its proximal end for injecting cooling water through the other end, an upper seal support attached inside the cap, a lower seal support attached to the head casing, an upper seal means in the form of an annular member having an inner and an outer periphery, and arranged around the bur sleeve (8, 9) with said inner periphery press-contacted onto the outer surface of said bur sleeve (8, 9) and with said outer periphery supported on the upper seal support, so as to seal a space between the upper region of the bur sleeve (8, 9) and the upper bearing, and a lower seal means in the form of an annular member having an inner and an outer periphery, and arranged around the bur sleeve (8, 9) with said inner periphery press-contacted onto the outer surface of said bur sleeve (8, 9) and with said outer periphery supported by the lower seal support and the lower end of the head housing, so as to seal a space between the lower region of the bur sleeve (8, 9) and the lower bearing.