To provide a hot water boiler with inexpensive fuel costs, superior in functions such as carrying out complete combustion of woody powder and grain.

The hot water boiler is composed of a woody granular object input opening 12, a combustion mechanism part 10 provided with a first jacket chamber 15 having a first hot water jacket 16 right above a substantially cylindrical combustion chamber 11 having a ventilation opening 14 communicated with a compulsive fan 13, a dust collection mechanism part 20 provided with a second jacket chamber 24 having a cylindrical inner tube 22 right above a substantially cylindrical dust collection chamber 21 provided adjacent to the combustion mechanism part 10, communicated with the first jacket chamber 15 by a connection pipe 23, and having a second hot water jacket 25 communicated with the first hot water jacket 16, a fuel supply mechanism part 30 provided adjacent to the combustion mechanism part 10 and automatically supplying woody granular object fuel F to the woody granular object input opening 12, and a communication mechanism part 40 communicating the first hot water jacket 16 and/or the second hot water jacket 25 with a facility H to be heated/dried.