The major objective of this study was to explore the economic usefullness of agricultural machine. For this objective, it seems reasonable that, to begine with, the specific feature of family farm should be defined. Family farm is a complete economic unit by itself in which business and household are interwined with production and consumption. Although farm management has been mentioned as a unit of firm-business, it has not been an independent unit but an interdependent unit. On the interdependence between a business firm and a consuming household, Dr E. O. Heady and his collaborators have represented excellent works. It is their fundamental view that the family farm pursues as a final goal a high level of living and a maximum satisfaction for family. On grounds of their theories, the writer hopes to analyse the effect of mechanization on the farmer's living. In fig. 4～6, the writer has indicated monetary income and consumption on the vertical axis, and non-monetary income and consumption on the horizontal axis. The curve mn means the production opportunity with limited resources and unchanged farming. The curve ID stands for the consumer indifference curves which illastrate individual's preference between monetary and non-monetary income at one point in time. Thus, the results obtained are summerised as follows: (1) When monetary income is decreased by the effect of mechanization, it does not necessary follow that the satisfaction of farm family decreases. If added non-monetary income is more than enonuh to compensate the loss of amount of money income, the farm family naturally obtain more satisfaction than before. (See fig. 4) (2) When monetary income is unchanged after mechanization occured, satisfaction of family will increase. (See fig. 5) (3) When monetary income is increased by the effect of mechanization, the satisfaction of family will increase. But the satisfaction of family will decrease, if excessive intensification or over-diversified farming sacrifice more non-monetary income than expected to the added money income. (See fig. 6) After the war, agricultural mechanization in Japan has developed rapidly. We can more easily understand the cause of this developement from our theory. In the study of the productivity of agricultural machine, it may be necessary to analyse this problem not only from the aspect of firm business but also from the aspect of firm-household interdependency.