DESCRIPTION , DPF REGENERATION CONTROL DEVICE, DPP RENERATION CONTROL METHOD, AND DPF REGENERATION SUPPORT SYSTEM TECHNICAL FIELD [0001] The present invention relates to a regeneration control device, a regeneration control method, and a regeneration support system for a diesel particulate filter (to be abbreviated to DPF hereafter) that collects particulate matter (exhaust gas particulate matter; to be abbreviated to PM hereafter) contained in exhaust gas from a diesel engine, and more particularly to a regeneration control device, a regeneration control method, and a regeneration support system for estimating an accumulated amount of ash accumulated in the DPF and performing ash washings BACKGROUND ART [0002].. A DPF is known as an effective technique for reducing PM discharged from a diesel engine. A bPF is a PM collecting device that uses a filter, and both a soot component and an ash component discharged from the engine accumulate in the DPF, The soot component can be burned away by forcible regeneration, but the ash cannot be burned away by forcible regeneration and therefore accurnulaLes gradually in the DPF. When the ash accumulates, the DPF be6omes blocked, leading to an increase in' exhaust gas pressure, and therefore an ash accumulation amount must be predicted periodically so that ash washing can be performed. [0003] Ash is generated when engine oil enters a combustion chamber through a gap between a cylinder and a piston and is burned therein. It is therefore known that the ash accumulation amount correlates with an engine oil consumption amount. [0004] Various techniques are available as ash accumulation amount estimating methods. For example, Japanese Patent Application Publication No. 2003-83036 (Patent Document 1) discloses a technique of determining the ash accumulation amount from a DPF differential pressure immediately after DPF regeneration is complete, Japanese Patent Publication No. 4032849 (Patent Document 2) discloses a technique of determining the ash accumulation amount by integrating a traveled distance, Japanese Patent Publication No. 3951618 (Patent Document 3) discloses a technique of determining an ash discharge amount from a map of an engine rotation speed and a fuel consumption amount and determining the accumulation amount by integrating the determined ash discharge amount, and Japanese Pateni Application Publication No. 2006-29326 (Patent 2 Document 4) discloses a technique of detecting an oil level r using an oil sensor and calculating the ash accumulation amount by integrating the detected oil level. [0005] Patent Document l: Japanese Patent Application Publication No. 2003-83036 Patent Document 2e Japanese Patent Publication No. 4032849 Patent Document 3e Japanese Patent Publication No. 3951618 Patent Document 4e Japanese Patent Application Publication No. 2006-29326 [0006] However, with the technique of determining the ash accumulation amount from the DPF differential pressure immediately after DPF regeneration is complete, disclosed in Patent Document 1, the DPF differential pressure does not always increase in proportion to an increase in the ash accumulation amount, and in a condition where no soot has accumulated, the DPF differential pressure substantially does not increase relative to ash accumulation. It is therefore°difficult to predict the ash accumulation amount alone from the DPF differential pressure with a high degree of precision. [0007] Further, when soot accumulates in,a DPF in which ash has accumulated, the DPF differential pressure is higher than in a case where ash has not accumulated, but since the amount of accumulated soot cannot be separated from the ash. accumulation amount, it is difficult to estimate the ash accumulation amount quantitatively with a high degree of accuracy. [0008] In the case of the technique of determining the ash accumulation amount by integrating the traveled distance, disclosed in Patent Document 2, the method is simpler. hut conditions such as an engine load cannot be taken into account, making it difficult to achieve estimation precision. With the technique of determining the ash discharge amount from the map of the engine rotation speed and the fuel consumption amount and determining the accumulation amount by integrating the determined ash discharge amount, disclosed in Patent Document 3, a large number of test points, and accordingly a large amount of time, are required to create the map, and it is therefore difficult to secure precision over the entire maps Further, with the technique of detecting the oil level using an oil sensor and calculating the ash accumulation amount by integrating the detected oil level, disclosed in Patent Document 4, the need to provide the oil sensor leads to an increase in cost. [0009] Hence, various methods exist for estimating the ash accumulation amount, but at the same time, problems remain. It is therefore necessary to provide a simple method of estimating the ash accumulation amount precisely without an increase in cost. Furthermore, ash, in contrast to soot, does not have carbon as a main component, and instead mainly contains components derived from metal-based additives contained in the engine oil. Therefore, the ash cannot be incinerated by combustion end must instead be washed, i. e. blown away by compressed air or the like. For this purpose, an operation must be performed in a dedicated service station having a washing facility. Hence, a DPF regeneration support system that can issue an ash washing request to an operator accurately so that ash washing can be performed efficiently at a service station is also required. DISCLOSURE OF THE INVENTION [0010] The present invention has been designed in consideration of these problems, and an object thereof is to"provide a DPF regeneration control device, a DPF regeneration control method, and a DPF regeneration support system with which an ash accumulation amount can be estimated precisely using a simple method and an ash washing request can be issued to an operator accurately so that ash washing can be performed efficiently at a service station. 5 [0011] To solve the problems described above, a DPP regeneration control device according to a first invention is a DPF regeneration control device for a diesel particulate filter (DPF) that is provided in an exhaust passage and collects exhaust gas particulate matter (PM), the DPF regeneration control device including: a forcible regeneration unit which forcibly regenerates the PM collected in the DPF, a differential pressure detection unit which detects a differential pressure between a front and a rear of the DPF; a DPF differential pressure setting unit which sets a DPF differential pressure generated in accordance with a total accumulated amount of soot and ash in advance through an experiment, or a calculation, sets a DPF differential pressure generated when an ash accumulation amount corresponds to an accumulation amount at which washing is required, as a washing request threshold, and sets a DPF differential pressure generated when the ash accumulation amount is larger than the accumulation amount at which washing is required such, that a reduction in output is