PURPOSE: To make improvement of operating efficiency with interchangeability with various working equipment possible by a method wherein one of a pair of holding parts respectively provided to a mobile truck and to a working equipment that is attachable thereto and detachable therefrom is placed on the same height whereat engagement becomes feasible by moving the self-propelled carriage.

CONSTITUTION: Holding parts 5 and 6 are provided to a frame 4 of a self-propelled carriage 1 which is made by removing a load-bearing bed from a machine like a crawler, and a pair of holding mechanism composed of holding parts 18 and 20 provided to a working equipment 11 for engagement ot and disengagement from the above-mentioned holding parts is prepared in a plurality by number. One pair of these pairs is positioned in the same height whereat the engagement becomes feasible by moving the self-propelled carriage 1, and the engagement is made with the holding parts 5 and 6 on the self-propelled carriage 1 and the holding parts 18 and 20 of the working equipment 11 engaged with each other. Next, another pair of the holding mechanism is made, by manual operation or operation by an actuator, to engage and a working equipment 11 is installed therewith to the self-propelled carriage 1. Thereby operating efficiency can be improved by employing various working equipment in turn.