A game system comprising game apparatus having an operating means (22, 26) with which a player makes an operation input, a display means for displaying a game image, and a storage means for storing in advance data for generating at least a virtual three-dimensional space, comprising: an input detecting means (36, S51) for detecting operation input data corresponding to an operation input from said operating means (22, 26), and temporarily storing it in said storage means; a player character controlling means (36, S5) for arranging a player character to be operated by the player in said virtual three-dimensional space by utilizing player character data stored in said storage means in response to a first operation input from said operating means (22,26) temporarily stored as operation input data by said input detecting means (36, S51); an object controlling means (36, S7) for arranging an object not to be operated by the player in said virtual three-dimensional space by utilizing object data stored in said storage means; a contact detection means (36, S9) for detecting that said player character and a predetermined object are contacted with each other; a viewpoint controlling means (36, S3) for controlling a viewpoint in said virtual three-dimensional space; and a viewpoint switching means (36, S21, S23, S29) for switching said viewpoint depending on a second operation input from said operating means (22, 26) temporarily stored as operation input data by said input detecting means(36, S51), wherein said contact detection means (36, S9) changes the contact detection data set used for contact detection between said player character and said predetermined object depending on a switching result of said viewpoint switching means (36, S21, S23, S29) characterized in that the viewpoint switching means (36, S21, S23, S29) is adapted to switch the viewpoint between a three-dimensional game screen and a two-dimensional game screen, and the contact detection data set includes: first detection data for performing a contact detection in all three dimensions of the virtual three-dimensional space; and second detection data for performing a contact detection in one dimension or a combination of any two dimensions of the virtual three-dimensional space, and said contact detection means (36, S9) selects any one of said first detection data and said second detection data depending on a switching result of said viewpoint switching means (36, S21, S23, S29) such that the first detection data is used when the three-dimensional game screen is displayed and the second detection data is used when the two-dimensional game screen is displayed. an input detecting means (36, S51) for detecting operation input data corresponding to an operation input from said operating means (22, 26), and temporarily storing it in said storage means; a player character controlling means (36, S5) for arranging a player character to be operated by the player in said virtual three-dimensional space by utilizing player character data stored in said storage means in response to a first operation input from said operating means (22,26) temporarily stored as operation input data by said input detecting means (36, S51); an object controlling means (36, S7) for arranging an object not to be operated by the player in said virtual three-dimensional space by utilizing object data stored in said storage means; a contact detection means (36, S9) for detecting that said player character and a predetermined object are contacted with each other; a viewpoint controlling means (36, S3) for controlling a viewpoint in said virtual three-dimensional space; and a viewpoint switching means (36, S21, S23, S29) for switching said viewpoint depending on a second operation input from said operating means (22, 26) temporarily stored as operation input data by said input detecting means(36, S51), wherein said contact detection means (36, S9) changes the contact detection data set used for contact detection between said player character and said predetermined object depending on a switching result of said viewpoint switching means (36, S21, S23, S29) characterized in that the viewpoint switching means (36, S21, S23, S29) is adapted to switch the viewpoint between a three-dimensional game screen and a two-dimensional game screen, and the contact detection data set includes: first detection data for performing a contact detection in all three dimensions of the virtual three-dimensional space; and second detection data for performing a contact detection in one dimension or a combination of any two dimensions of the virtual three-dimensional space, and said contact detection means (36, S9) selects any one of said first detection data and said second detection data depending on a switching result of said viewpoint switching means (36, S21, S23, S29) such that the first detection data is used when the three-dimensional game screen is displayed and the second detection data is used when the two-dimensional game screen is displayed. first detection data for performing a contact detection in all three dimensions of the virtual three-dimensional space; and second detection data for performing a contact detection in one dimension or a combination of any two dimensions of the virtual three-dimensional space, and said contact detection means (36, S9) selects any one of said first detection data and said second detection data depending on a switching result of said viewpoint switching means (36, S21, S23, S29) such that the first detection data is used when the three-dimensional game screen is displayed and the second detection data is used when the two-dimensional game screen is displayed.