Mokudai was a deputy, such as a retainer, sent to the post in behalf of an appointed provincial governor who remained at his place from mid Heian period to Kamakura period. It was also know as gandai.
It turned into a government official position, such as a local governor, sent in behalf of an officer who was supposed to leave the capital and work at the place. Here, the above mentioned deputy of the yonin (remote appointments) provincial governor from the middle of the Heian period to the Kamakura period is described.
The mokudai as a provincial governor's private deputy existed in the Nara period. However, in the middle of the 10th century, zaichokanjin (the local officials in Heian and Kamakura periods) started to manage kokuga (provincial government offices). More and more appointed officials did not move to ningoku (place of appointment) and delegated to zaichokanjin. It was called yonin (remote appointments). A yonin provincial governor sent his retainer to the place as a deputy and let him supervise the zaichokanjin. This deputy was called mokudai.
Those who were capable with preparing official documents, etc. were appointed to the mokudai. Moreover, geki of daijokan (secretary of the grand council) might be sent by a chigyo-kokushu (provincial proprietor), an influential kugyo (top court official). However, when armed groups rose in various places in the end of the Heian period, chigyo-kokushu (provincial proprietor) and zuryo (the head of the provincial governors) began to send those who were excellent in military affairs to confront them. The zaichokanjin dealt with tax collection, military affairs, etc. under the supervision of a mokudai in the kokuga.
Gandai in samurai class
Moreover, among the samurai class, the local governor might be called gandai or mokudai. The article of November 22 (lunar calendar), 1256 of "Azuma Kagami" (The Mirror of the East) states that 'However, they may remain gandai only in the head-of-the-family (Tokimune HOJO)'s childhood' when Tokiyori HOJO became a priest and handed the regency to Nagatoki HOJO. According to "Buke Myomoku-sho" (compilation of historical data of the Edo period), the samurai class used gandai in order to distinguish it from court noble's mokudai. However, both mokudai and gandai were used by court nobles and the samurai in actual documents.