The suspension mechanism (80) is movably attached to the attachment shaft (82) and has a cylindrical body (84) having a hemispherical outer peripheral surface (841), and a hemispherical shape of the cylindrical body provided on the chassis. A first mounting shaft holding portion (12a1) that holds the mounting shaft in a swingable manner by bringing the outer peripheral surface into contact with the hemispherical inner surface (12a11), and a smooth shaft-shaped lower end of the shaft provided at the lowermost portion of the mounting shaft The portion (821) and the first attachment shaft holding portion are provided on the chassis, and have a substantially conical apex-corresponding portion (12a21) and a slope-corresponding portion (12a22), and the bottom end portion of the smooth-shaped shaft is the apex. The second mounting shaft holding portion (12a2) that holds the mounting shaft in contact with the corresponding portion or the inclined surface corresponding portion, and the urging member that urges the cylindrical body 84 and the shaft lower end portion (84) to be separated from each other. (86).サスペンション機構（８０）が、取り付け軸（８２）に移動可能に取り付けられると共に、半球面形状の外周面（８４１）を有する筒体（８４）と、シャシに設けられ、筒体の半球面形状の外周面を半球面形状の内面（１２ａ１１）に当接させて取り付け軸を揺動可能に保持する第１取り付け軸保持部（１２ａ１）と、取り付け軸の最下部に設けられる滑面形状の軸下端部（８２１）と、第１取り付け軸保持部の下方でシャシに設けられ、大略円錐形状の頂点相当部（１２ａ２１）と斜面相当部（１２ａ２２）を有し、滑面形状の軸下端部を頂点相当部または斜面相当部に当接させて取り付け軸を保持する第２取り付け軸保持部（１２ａ２）と、筒体８４と軸下端部（８４）を相互に離間するように付勢する付勢部材（８６）とを備える。
